[C/CPP系列知识] 那些程序C语言可以编译通过但C++无法编译成功 Write a C program that won’t compile in C++

http://www.geeksforgeeks.org/write-c-program-wont-compiler-c/

1) C++中在函数声明之前调用一个函数会引发错误，但是在C中有可能可以。 参考 http://www.cnblogs.com/diegodu/p/4580292.html

下面的程序可以用gcc编译，但g++无法编译。

#include<stdio.h>
int main()
{
   foo(); // foo() is called before its declaration/definition
} 
 
int foo()
{
   printf("Hello");
   return 0; 
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test3.c 
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test3.c 
test3.c: In function 'int main()':
test3.c:4:9: error: 'foo' was not declared in this scope
     foo(); // foo() is called before its declaration/definition

2) C++中将一个非const指针指向一个const变量是非法的，但在C中是可以的。

#include <stdio.h>
 
int main(void)
{
    int const j = 20;
 
    /* The below assignment is invalid in C++, results in error
       In C, the compiler *may* throw a warning, but casting is
       implicitly allowed */
    int *ptr = &j;  // A normal pointer points to const
 
    printf("*ptr: %d
", *ptr);
 
    return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test4.c 
test4.c: In function 'main':
test4.c:10:16: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
     int *ptr = &j;  // A normal pointer points to const
                ^
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test4.c 
test4.c: In function 'int main()':
test4.c:10:17: error: invalid conversion from 'const int*' to 'int*' [-fpermissive]
     int *ptr = &j;  // A normal pointer points to const
 　　             ^

3) C中可以将void* 赋值给其他的指针，如int*， char*等，但是在C++中则需要显示的类型转换。

#include <stdio.h>
int main()
{
   void *vptr;
   int *iptr = vptr; //In C++, it must be replaced with int *iptr=(int *)vptr; 
   return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test5.c 
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test5.c 
test5.c: In function 'int main()':
test5.c:5:17: error: invalid conversion from 'void*' to 'int*' [-fpermissive]
     int *iptr = vptr; //In C++, it must be replaced with int *iptr=(int *)vptr;

这也就意味着在C++中我们调用malloc时需要显示的转换类型，“int *p = (void *)malloc(sizeof(int))，而在C中不需要。

4) C++中const变量声明的同时必须赋初值，但C中不需要赋初值。

#include <stdio.h>
int main()
{
    const int a;   // LINE 4
    return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test6.c 
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test6.c 
test6.c: In function 'int main()':
test6.c:4:15: error: uninitialized const 'a' [-fpermissive]
     const int a;   // LINE 4
               ^

5) C++中的关键字在C中不可用，如new，delete，explicit等。

#include <stdio.h>
int main(void)
{
    int new = 5;  // new is a keyword in C++, but not in C
    printf("%d", new);
}

6) C++有着更加严格的类型检查。C++无法通过编译，有error，C中仅仅报warning

#include <stdio.h>
int main()
{
    char *c = 333;
    printf("c = %u", c);
    return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test7.c 
test7.c: In function 'main':
test7.c:4:15: warning: initialization makes pointer from integer without a cast [enabled by default]
     char *c = 333;
               ^
test7.c:5:5: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'char *' [-Wformat=]
     printf("c = %u", c);
     ^
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test7.c 
test7.c: In function 'int main()':
test7.c:4:15: error: invalid conversion from 'int' to 'char*' [-fpermissive]
     char *c = 333;
               ^
test7.c:5:23: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'char*' [-Wformat=]
     printf("c = %u", c);
                       ^

7) C++变量可以在任意位置定义，但是C中必须是一个语句块开始的地方。（gcc 中可以编译。。）

#include <stdio.h>
int main()
{
    int i;
    i=5;
    i++;
    int j;
    return 0;
}

8) C++存在loop variable，C中不存在。

#include <stdio.h>
int main()
{
for(int i=0; i<5; i++)
  ;  
    
return 0;
}

9) C++ main函数必须返回int，C可以返回void

#include <stdio.h>
void main (int argc, char *argv[])
{
     printf("bye
");
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test9.c 
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test9.c 
test9.c:2:34: error: '::main' must return 'int'
 void main (int argc, char *argv[])
                                  ^
diego@ubuntu:~/myProg/geeks4geeks/cpp$