定义
const定义常变量,变量变成只读的权限(即不能作为左值)
使用
1. 数据类型对于const透明,const int == intconst
eg:
2. const直接修饰的内容不能做左值
eg:
int main()
{
int a = 10;
int b =20;
const int ca =100;
int *p =&a;
*p = 100;
p =&b;
const int *cp1 =&a;
*p1 =100; //error
cp1 =&b; //ok
int const *cp2 =&a; //cp2==cp1
int*const cp3 =&a;
*cp3 =100; //ok
cp3 =&b; //error
int const*const cp4 = &a; //error限制太多,无意义
*cp =100; //error
cp4 =&b; //error
}
3.权利的传递(权利可同等传递,可缩小,但不可放大)
eg:
int main()
{
int a = 10;
int b = 10;
const int ca = 100;
int *p = &a;
p =&ca; //error因为*p会将ca的值修改,所以错误 eg:*p = 200;
const int *cp1 =&a; //error
cp1 = &ca; //okcp1可修改成&ca,因为cp1不允许解引用,所以改变不了ca的值
*cp1 = 200; //error
int *const cp2 =&a; //ok
int *const cp3 =&ca; //error 因为const int ca =100;而*cp3会修改ca的值,所以错误
return 0;
}
Tips
1.读/写权限
int main()
{
int a=10;
b=a; //a作右值,使用读权限
a=100; //a作左值(在赋值符号左边),使用写权限
}
const定义常变量,变量变成只读的权限(即不能作为左值)
使用
1. 数据类型对于const透明,const int == intconst
eg:
int main()
{
const int ca =100; //error只读权限,ca不能做左值
ca = 100; //error
b = ca; //ok
if(ca <b) //ok
const int cb; //errorcb变量,需要给cb设置初始值
cb = 100;
//error
int const cb = 100; //ok
cb= 200; //error;
b = cb; //ok
}
2. const直接修饰的内容不能做左值
eg:
int main()
{
int a = 10;
int b =20;
const int ca =100;
int *p =&a;
*p = 100;
p =&b;
const int *cp1 =&a;
*p1 =100; //error
cp1 =&b; //ok
int const *cp2 =&a; //cp2==cp1
int*const cp3 =&a;
*cp3 =100; //ok
cp3 =&b; //error
int const*const cp4 = &a; //error限制太多,无意义
*cp =100; //error
cp4 =&b; //error
}
3.权利的传递(权利可同等传递,可缩小,但不可放大)
eg:
int main()
{
int a = 10;
int b = 10;
const int ca = 100;
int *p = &a;
p =&ca; //error因为*p会将ca的值修改,所以错误 eg:*p = 200;
const int *cp1 =&a; //error
cp1 = &ca; //okcp1可修改成&ca,因为cp1不允许解引用,所以改变不了ca的值
*cp1 = 200; //error
int *const cp2 =&a; //ok
int *const cp3 =&ca; //error 因为const int ca =100;而*cp3会修改ca的值,所以错误
return 0;
}
Tips
1.读/写权限
int main()
{
int a=10;
b=a; //a作右值,使用读权限
a=100; //a作左值(在赋值符号左边),使用写权限
}