c++面试题

#include<iostream>
#include<cstdio>
using namespace std;
char * str() {
  char a[6]="12345";//warning: address of local variable ‘a’ returned [-Wreturn-local-addr]
  return &a[0];
}
void arraySize(char *a) {
  printf("%d %p
",sizeof(a),a);//8 0x7ffda5e6aeb0
  // warning: ‘sizeof’ on array function parameter ‘a’ will return size of ‘char*’ [-Wsizeof-array-argument]
  a = "aaaa"; //warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
}
//数组作为参数传过来，无论接收的是 char* ,char[]还是char[x]，这些都是char*,并且可修改
void arraySize1(char a[]) {//note: declared here
  printf("%d %p
",sizeof(a),a);//8 0x7ffda5e6aeb0
  a = "bbbb"; // warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
}
void arraySize2(char a[10]) {//note: declared here
  printf("%d %p
",sizeof(a),a);//8 0x7ffda5e6aeb0
  a = "bbbb";
  //warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long unsigned int’ [-Wformat=]
}
void arraySize3(char *&a) {
  printf("%d %p
",sizeof(a),a);
  a = "aaaa";
}
struct node {
  char *a;
  char b; 
};
struct node1 {
  char *a;
  char b;
  short c;
};
struct node2 {
  char *a;
  char b;
  short c;
  int d;
};
struct node3 {
  char *a;//50
  char b;//58
  short c;//5a
  int d;//5c
  char e;//60
};
struct node4 {
  int a;
  int b;
};
struct node5 {
  int a;
};
struct node6 {
  int a;
  char *b;
};
struct node7 {
  char a;
};
struct node8 {
  short a;
};
struct node9 {
  char *a;//70
  char b;//78
  char c;//79
  int d;//7c
  char e;//80
};
struct nodea {
  char *a;//90
  char b;//98
  char c;//99
  int d;//9c
  char e;//a0
  char f;//a1
};
struct nodeb {
  char *a;//90
  char b;//98
  char c;//99
  char d;//9a
  char e;//9b
  int f;//9c
};
int main() {
  cout<<sizeof(node)<<endl;//16
  cout<<sizeof(node1)<<endl;//16
  cout<<sizeof(node2)<<endl;//16
  cout<<sizeof(node3)<<endl;//24
  node3 u;
  printf("%p %p %p %p %p
",&u.a,&u.b,&u.c,&u.d,&u.e);
  //0x7fffc7584050 0x7fffc7584058 0x7fffc758405a 0x7fffc758405c 0x7fffc7584060
  cout<<sizeof(node4)<<endl;//8
  cout<<sizeof(node5)<<endl;//4
  cout<<sizeof(node6)<<endl;//16
  cout<<sizeof(node7)<<endl;//1
  cout<<sizeof(node8)<<endl;//2
  cout<<sizeof(node9)<<endl;//24
  node9 v;
  printf("%p %p %p %p %p
",&v.a,&v.b,&v.c,&v.d,&v.e);
  //0x7fffc7584070 0x7fffc7584078 0x7fffc7584079 0x7fffc758407c 0x7fffc7584080
  cout<<sizeof(nodea)<<endl;//24
  nodea w;
  printf("%p %p %p %p %p %p
",&w.a,&w.b,&w.c,&w.d,&w.e,&w.f);
  //0x7fffc7584090 0x7fffc7584098 0x7fffc7584099 0x7fffc758409c 0x7fffc75840a0 0x7fffc75840a1
  cout<<sizeof(nodeb)<<endl;//16
  nodeb x;
  printf("%p %p %p %p %p %p
",&x.a,&x.b,&x.c,&x.d,&x.e,&x.f);
  //0x7ffe4cab4b30 0x7ffe4cab4b38 0x7ffe4cab4b39 0x7ffe4cab4b3a 0x7ffe4cab4b3b 0x7ffe4cab4b3c
  //struct 元素地址安排：找出成员的最大长度，然后所有成员按顺序和最大成员对齐
  //8
  //44
  //2222
  //11111111
  //224
  //1124
  //上面是部分可能的排列，但11411是不会出现的，4要么在最左要么在最右
  //所以nodea和nodeb虽然成员元素类型一样,但顺序不一样的话结构体大小也不一样
  //nodea
  //8     char*
  //11_4  char char int
  //11_   char char
  //nodeb
  //8     char*
  //11114 char char char char int
  cout<<"-----------sizeof---------"<<endl;
  cout<<sizeof(void)<<endl;//1  warning: invalid application of ‘sizeof’ to a void type [-Wpointer-arith]
  cout<<sizeof(char)<<endl;//1
  cout<<sizeof(short)<<endl;//2
  cout<<sizeof(int)<<endl;//4
  cout<<sizeof(long)<<endl;//8
  cout<<sizeof(long int)<<endl;//8
  cout<<sizeof(float)<<endl;//4
  cout<<sizeof(double)<<endl;//8
  cout<<sizeof(long double)<<endl;//16
  cout<<sizeof(char*)<<endl;//8
  cout<<sizeof(void*)<<endl;//8
  char *s1 = "12345";//都指向常量内存区的"12345" 
                     //warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
  char *s2 = "12345";//都指向常量内存区的"12345"
                     //warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
  char a1[] = "12345";//有额外分配内存，可修改
  char a2[] = "12345";//有额外分配内存，可修改
  char b1[6] = "12345";//有额外分配内存，可修改
  char b2[6] = "12345";//有额外分配内存，可修改
  const char *c1 = "12345";//都指向常量内存区的"12345"
  const char *c2 = "12345";//都指向常量内存区的"12345"
  cout<<(s1==s2)<<" "<<sizeof(s1)<<" "<<sizeof(s2)<<endl;//1 8 8
  cout<<(a1==a2)<<" "<<sizeof(a1)<<" "<<sizeof(a2)<<endl;//0 6 6
  cout<<(b1==b2)<<" "<<sizeof(b1)<<" "<<sizeof(b2)<<endl;//0 6 6
  cout<<(c1==c2)<<" "<<sizeof(c1)<<" "<<sizeof(c2)<<endl;//1 8 8
  cout<<(s1==c1)<<endl;//1
  cout<<(a1==b1)<<endl;//0
  printf("%p
",s1);//0x400e74
  printf("%p
",s2);//0x400e74
  printf("%p
",a1);//0x7ffe8f1acf40
  printf("%p
",a2);//0x7ffe8f1acf50
  printf("%p
",b1);//0x7ffe8f1acf60
  printf("%p
",b2);//0x7ffe8f1acf70
  printf("%p
",c1);//0x400e74
  printf("%p
",c2);//0x400e74
  printf("%p
",str());//(nil)
  //p1,p3是const char*类型，p2是char* 类型，p1,p2,*p3可变，*p1,*p2,p3不可变
  const char* p1 = "12345";
  char const *p2 = "12345";//warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
  char* const p3 = "12345";
  p1 = NULL;
  printf("p1:%p
",p1);
  //*p1 = NULL;  //error: assignment of read-only location ‘* p1’
  p2 = NULL;
  printf("p2:%p
",p2);
  //*p2 = NULL;  //error: assignment of read-only location ‘* p2’
  //p3 = NULL;  //error: assignment of read-only variable ‘p3’
  //*p3 = NULL; //warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
                //段错误 (核心已转储) 修改了常量指针
  //printf("p3:%p
",p3);
  const char *p4 = "12345"; //*p是const,p可变
  p4 = NULL;
  printf("p4:%p
",p4);
  //*p4 = NULL;  //error: assignment of read-only location ‘* p4’
  //const (char *) p5 = "12345";//p是const,*p可变, error: expected primary-expression before ‘const’
  char* const p6 = "12345"; //p是const,*p可变
  //p6 = NULL;  // error: assignment of read-only variable ‘p6’
  //*p6 = NULL;//段错误 (核心已转储) 修改了常量指针
  //printf("p6:%p
",p6);
  const char* const p7 = "12345"; //p和*p都是const
  //p7 = NULL;  //error: assignment of read-only variable ‘p7’
  //*p7 = NULL;  //error: assignment of read-only location ‘*(const char*)p7’
  char const * p8 = "12345";// *p是const,p可变
  p8 = NULL;
  printf("p8:%p
",p8);
  //*p8 = NULL;  //error: assignment of read-only location ‘* p8’
  //(char*) const p9 = "12345";//p是const,*p可变,error: expected primary-expression before ‘const’
  char const* const pa = "12345";// p和*p都是const
  //pa = NULL;  //error: assignment of read-only variable ‘pa’
  //*pa = NULL;  //error: assignment of read-only location ‘*(const char*)pa’
  char a[8]={0x01,0x23,0x45,0x67,0x89,0xab,0xcd,0xef};
  short *b = (short*)a;
  cout<<hex<<"{"<<b[0]<<","<<b[1]<<","<<b[2]<<","<<b[3]<<"}"<<endl;//{2301,6745,ab89,efcd}
  //printf("{%x,%x,%x,%x}
",b[0],b[1],b[2],b[3]);
  int *c = (int*)a;
  printf("{%x,%x}
",c[0],c[1]);//{67452301,efcdab89}
  char tmp[10]="12345";//tmp是常量类型，不能修改
  char *tmp1;
  char tmp2[10]="12345";
  tmp1 = tmp;
  printf("%d %p
",sizeof(tmp),tmp);//10 0x7ffe91a345e0
  arraySize(tmp);//8 0x7ffe91a345e0
  printf("%s %p
",tmp,tmp);//12345 0x7ffe91a345e0
  arraySize1(tmp);//8 0x7ffe91a345e0
  printf("%s %p
",tmp,tmp);//12345 0x7ffe91a345e0
  arraySize2(tmp);//8 0x7ffe91a345e0
  printf("%s %p
",tmp,tmp);//12345 0x7ffe91a345e0
  //arraySize3(tmp);//error: invalid initialization of non-const reference of type ‘char*&’ from an rvalue of type ‘char*’
  char *tmp3[] = {"12345","23456"};//warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
  printf("%d %p %p %p %s %s %s
",sizeof(tmp3),tmp3,tmp3[0],tmp3[1],tmp,tmp3[0],tmp3[1]);
  //16 0x7ffe640d66f0 0x401833 0x40186a 12345 12345 23456
  char **tmp4;
  printf("%p
",tmp4);//(nil)
  //*tmp4 = "12345";//warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
                    //段错误 (核心已转储) *tmp4 -> *(nil)
  tmp4 = tmp3;
  printf("%p %p %p %c %d
",tmp4,*tmp4,**tmp4,**tmp4,sizeof(tmp4));//0x7fff24f47670 0x4017e3 0x31 1 8
  //tmp4是二维指针，值为"一维指针数组"首地址,指向存着一维指针的数组
  //*tmp4是一维指针，值为"数组"首地址,指向存着字符串
  //**tmp4是字符，值为字符'1'
  //tmp = tmp1;//error: incompatible types in assignment of ‘char*’ to ‘char [10]’
  //tmp2 = tmp;//error: invalid array assignment
  //tmp4 = tmp1;//error: cannot convert ‘char*’ to ‘char**’ in assignment
  tmp4 = (char**)tmp1;
  char tmp5[2][10] = {"12345","23456"};
  printf("%d %p %p %p %s %s %s
",sizeof(tmp5),tmp5,tmp5[0],tmp5[1],tmp,tmp5[0],tmp5[1]);
  //20 0x7ffe72775570 0x7ffe72775570 0x7ffe7277557a 12345 12345 23456
  //tmp3 = tmp5;// error: incompatible types in assignment of ‘char [2][10]’ to ‘char* [2]’
  char *tmp6[] = {tmp5[1],tmp5[2]};//[]优先级比*高，所以这里相当于(char*)tmp6[],是一个一维数组，元素为char*类型
  //tmp3 = {tmp5[1],tmp5[2]};//error: assigning to an array from an initializer list
  //tmp3 = tmp6;//error: invalid array assignment     tmp3不可更改,因为tmp3是数组，元素类型为char*
  return 0;
}

 c++常见面试题30道

如何理解const char*, char const*, char*const等

上面这些声明意义，及他们的区别你都记住了吗？反正我是记不住，打死也记不住。

听说Bjarne在他的The C++ Programming Language里面给出过一个助记的方法：

把一个声明从右向左读。

char * const cp; ( * 读成 pointer to) cp is a const pointer to char

const char * p; p is a pointer to constchar;

char const * p; 同上因为C++里面没有const*的运算符，所以const只能属于前面的类型。

常见C++面试题及基本知识点总结（一）

对于复杂的定义（函数或数组），通过运算符优先级分析，但运算符优先级不同版本太多，还是自己测一遍自己用的编译器优先级是怎样的比较好