• C++程序设计(第4版)读书笔记_C++概览:基础知识


    变量赋值

    常用的变量赋值都是用“=”去赋值的

    1 int i = 2;

    但是如果把一个浮点数赋值给i的话,就会造成精度损失,在C++中最好使用初始化列表的方式“{}”给变量赋值,这样可以保证不会发生某些可能导致信息丢失的类型转换

    1 #include <iostream>
    2 using namespace std;
    3 
    4 int main() {
    5     int i {2.3};
    6     return 0;
    7 }

    比如这样声明,编译器就会报错

    <source>: In function 'int main()':
    5 : <source>:5:12: error: narrowing conversion of '2.2999999999999998e+0' from 'double' to 'int' inside { } [-Wnarrowing]
    int i {2.3};
    ^
    Compiler exited with result code 1
     
    auto
    通过一个for循环来学习使用auto的方法
     1 #include <iostream>
     2 using namespace std;
     3 
     4 int main() {
     5     int v[] = {0, 1, 2, 3, 4};
     6 
     7     for (auto x : v) {
     8         cout << x << "	";
     9     }
    10     cout << endl;
    11     
    12     for (auto i = 0; i < sizeof(v) / sizeof(int); ++i) {
    13         cout << v[i] << "	";    
    14     }
    15     cout << endl;
    16     
    17     for (auto x : {0, 1, 2, 3, 4}) {
    18         cout << x << "	";
    19     }
    20     cout << endl;
    21     
    22     for (auto x : v) {
    23         x += 1;
    24     }
    25     for (auto x : v) {
    26         cout << x << "	";
    27     }
    28     cout << endl;
    29     
    30     /* 对于不带引用的情况,可以理解为对于v的每个元素将其从头到尾依次放入x并打印 */
    31     for (auto & x : v) {
    32         x += 1;
    33     }
    34     for (auto x : v) {
    35         cout << x << "	";
    36     }
    37     cout << endl;
    38     
    39     return 0;
    40 }

    运行结果如下:

    0	1	2	3	4	
    0	1	2	3	4	
    0	1	2	3	4	
    0	1	2	3	4	
    1	2	3	4	5

    update 2017-11-02:范围for循环对指针无效,因为我们不知道数组的大小了

    结构

    在结构里面我们先完成一个最简单的vector,再下面介绍的类中我们可以看到一些优化的部分,可以看到类的操作会更加简单,使用者根本就无需关注里面的实现。

    先来看结构实现的代码吧

     1 struct Vector 
     2 {
     3     int sz;
     4     double * elem;
     5 };
     6 
     7 void vector_init(Vector & v, int s)
     8 {
     9     v.elem = new double[s];
    10     v.sz = s;
    11 }
    12 
    13 double read_and_sum(int s)
    14 {
    15     Vector v;
    16     vector_init(v, s);
    17 
    18     for (int i = 0; i != s; ++i) {
    19         cin >> v.elem[i];
    20     }
    21 
    22     double sum = 0;
    23     for (int i = 0; i != s; ++i) {
    24         sum += v.elem[i];
    25     }
    26 
    27     return sum;
    28 }
    29 
    30 int main()
    31 {
    32     cout << read_and_sum(5) << endl;
    33     return 0;
    34 }

    运行结果:

    我们先简单的介绍一下类,在后面会不断的完善,这里我们就仅仅简单的引入构造函数、初始化列表、运算符重载的概念而已。 
    具体比上面结构的优点,我们还是看代码吧:
     1 #include <iostream>
     2 #include <new>
     3 
     4 using namespace std;
     5 
     6 class Vector 
     7 {
     8 public:
     9     /* 
    10         C++11这样用 
    11         低版本的话就是 Vector(int s) : elem(new double[s]), sz(s) {}
    12      */
    13     Vector(int s) : elem{new double[s]}, sz{s}
    14     {
    15 
    16     }
    17 
    18     double & operator[](int i)
    19     {
    20         return elem[i];
    21     }
    22 
    23     int size()
    24     {
    25         return sz;
    26     }
    27 
    28 private:
    29     double * elem;
    30     int sz;
    31 };
    32 
    33 double read_and_sum(int s)
    34 {
    35     Vector v(s);
    36 
    37     for (int i = 0; i != v.size(); ++i) {
    38         cin >> v[i];
    39     }
    40 
    41     double sum = 0;
    42     for (int i = 0; i != v.size(); ++i) {
    43         sum += v[i];
    44     }
    45 
    46     return sum;
    47 }
    48 
    49 int main()
    50 {
    51     cout << read_and_sum(5) << endl;
    52     return 0;
    53 }

    运行结果:

    枚举
    书上的枚举前面加了class
     1 #include <iostream>
     2 #include <new>
     3 
     4 using namespace std;
     5 
     6 enum class Traffic_light
     7 {
     8     green,
     9     yellow,
    10     red
    11 };
    12 
    13 
    14 Traffic_light & operator++(Traffic_light & t)
    15 {
    16     switch(t) 
    17     {
    18         case Traffic_light::green:
    19             return t = Traffic_light::yellow;
    20         case Traffic_light::yellow:
    21             return t = Traffic_light::red;
    22         case Traffic_light::red:
    23             return t = Traffic_light::green;
    24     }
    25 }
    26 
    27 int main()
    28 {
    29     Traffic_light src = Traffic_light::green;
    30 
    31     for (int i = 0; i < 3; ++i) {
    32         Traffic_light next = ++src;
    33         cout << next << endl;
    34     }
    35 
    36     return 0;
    37 }

    在编译的时候出错了:

    main.cpp: In function 'int main()':
    main.cpp:33:8: error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'Traffic_light')
       cout << next << endl;
    
       ~~~~~^~~~~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:108:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(__ostream_type& (*__pf)(__ostream_type&))
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:108:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}'
    /usr/local/include/c++/7.2.0/ostream:117:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
           operator<<(__ios_type& (*__pf)(__ios_type&))
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:117:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}'
    /usr/local/include/c++/7.2.0/ostream:127:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(ios_base& (*__pf) (ios_base&))
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:127:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::ios_base& (*)(std::ios_base&)'
    /usr/local/include/c++/7.2.0/ostream:166:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(long __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:166:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long int'
    /usr/local/include/c++/7.2.0/ostream:170:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(unsigned long __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:170:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long unsigned int'
    /usr/local/include/c++/7.2.0/ostream:174:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(bool __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:174:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'bool'
    In file included from /usr/local/include/c++/7.2.0/ostream:693:0,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:91:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]
         basic_ostream<_CharT, _Traits>::
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'short int'
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:181:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(unsigned short __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:181:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'short unsigned int'
    In file included from /usr/local/include/c++/7.2.0/ostream:693:0,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:105:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]
         basic_ostream<_CharT, _Traits>::
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'int'
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:192:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(unsigned int __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:192:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'unsigned int'
    /usr/local/include/c++/7.2.0/ostream:201:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(long long __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:201:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long long int'
    /usr/local/include/c++/7.2.0/ostream:205:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(unsigned long long __n)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:205:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long long unsigned int'
    /usr/local/include/c++/7.2.0/ostream:220:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(double __f)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:220:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'double'
    /usr/local/include/c++/7.2.0/ostream:224:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(float __f)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:224:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'float'
    /usr/local/include/c++/7.2.0/ostream:232:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(long double __f)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:232:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long double'
    /usr/local/include/c++/7.2.0/ostream:245:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
           operator<<(const void* __p)
           ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:245:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'const void*'
    In file included from /usr/local/include/c++/7.2.0/ostream:693:0,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:119:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
         basic_ostream<_CharT, _Traits>::
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:119:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}'
    In file included from /usr/local/include/c++/7.2.0/string:52:0,
                     from /usr/local/include/c++/7.2.0/bits/locale_classes.h:40,
                     from /usr/local/include/c++/7.2.0/bits/ios_base.h:41,
                     from /usr/local/include/c++/7.2.0/ios:42,
                     from /usr/local/include/c++/7.2.0/ostream:38,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/bits/basic_string.h:6082:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
         operator<<(basic_ostream<_CharT, _Traits>& __os,
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/bits/basic_string.h:6082:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and 'Traffic_light'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/bits/ios_base.h:46:0,
                     from /usr/local/include/c++/7.2.0/ios:42,
                     from /usr/local/include/c++/7.2.0/ostream:38,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/system_error:217:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)
         operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/system_error:217:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const std::error_code&'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:497:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)
         operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:497:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   deduced conflicting types for parameter '_CharT' ('char' and 'Traffic_light')
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:502:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)
         operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:502:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'char'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:508:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)
         operator<<(basic_ostream<char, _Traits>& __out, char __c)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:508:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'char'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:514:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)
         operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:514:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'signed char'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:519:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)
         operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:519:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'unsigned char'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:539:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)
         operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:539:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   mismatched types 'const _CharT*' and 'Traffic_light'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/ostream:693:0,
                     from /usr/local/include/c++/7.2.0/iostream:39,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:321:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)
         operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const char*'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:556:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)
         operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:556:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const char*'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:569:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)
         operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:569:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const signed char*'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:574:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)
         operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:574:5: note:   template argument deduction/substitution failed:
    main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const unsigned char*'
       cout << next << endl;
    
               ^~~~
    In file included from /usr/local/include/c++/7.2.0/iostream:39:0,
                     from main.cpp:1:
    /usr/local/include/c++/7.2.0/ostream:682:5: note: candidate: template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&)
         operator<<(_Ostream&& __os, const _Tp& __x)
         ^~~~~~~~
    /usr/local/include/c++/7.2.0/ostream:682:5: note:   template argument deduction/substitution failed:
    /usr/local/include/c++/7.2.0/ostream: In substitution of 'template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = Traffic_light]':
    main.cpp:33:11:   required from here
    /usr/local/include/c++/7.2.0/ostream:682:5: error: no type named 'type' in 'struct std::enable_if<false, std::basic_ostream<char>&>'
    main.cpp: In function 'Traffic_light& operator++(Traffic_light&)':
    main.cpp:25:1: warning: control reaches end of non-void function [-Wreturn-type]
     }
    
     ^
     
    应该是没有重载标准输出导致的,这个后续再完善吧,如果把class去掉,就可以正常运行了。囧……
     
    静态断言 static_assert
    static_assert能用于任何可以表达为常量表达式的东西,static_assert(A, S)的作用是当A不为true时把S作为一条编译器错误信息输出,这是C++11的特性。
    static_assert最重要的用途是为泛型编程中作为形参的类型设置断言。
     1 #include <iostream>
     2 #include <cassert>
     3 
     4 using namespace std;
     5 
     6 int main()
     7 {
     8     const int i = 5;
     9 
    10     static_assert(i < 4, "5 is larger then 4");
    11 
    12     return 0;
    13 
    14 }

    编译结果:

    main.cpp: In function 'int main()':
    main.cpp:10:9: error: static assertion failed: 5 is larger then 4
             static_assert(i < 4, "5 is larger then 4");
    
             ^~~~~~~~~~~~~

    如果上述代码中的i不为常量,那么编译就直接报错了

    main.cpp: In function 'int main()':
    main.cpp:10:5: error: non-constant condition for static assertion
         static_assert(i < 4, "5 is larger then 4");
    
         ^~~~~~~~~~~~~
    main.cpp:10:5: error: the value of 'i' is not usable in a constant expression
    main.cpp:8:9: note: 'int i' is not const
         int i = 5;
    
             ^
    另外可以用static_assert来判断编译的平台,达到尽早检查的目的
     1 #include <iostream>
     2 #include <cassert>
     3 
     4 using namespace std;
     5 
     6 int main()
     7 {
     8     static_assert(sizeof(void *) == 4, "64-bit code generation is not supported.");
     9     
    10     const int i = 5;
    11 
    12     static_assert(i < 4, "5 is larger then 4");
    13 
    14     return 0;
    15 
    16 }

    编译结果:

    main.cpp: In function 'int main()':
    main.cpp:8:5: error: static assertion failed: 64-bit code generation is not supported.
         static_assert(sizeof(void *) == 4, "64-bit code generation is not supported.");
    
         ^~~~~~~~~~~~~
    main.cpp:12:5: error: static assertion failed: 5 is larger then 4
         static_assert(i < 4, "5 is larger then 4");
    
         ^~~~~~~~~~~~~
     
     
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  • 原文地址:https://www.cnblogs.com/abc-begin/p/7765669.html
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