C++：通过指针调用成员函数
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C++: Calling member function via pointer
提问by ashur
I have this example code of using pointer to member function, which I want to change during runtime, but I cannot make it work. I've already tried this->*_currentPtr(4,5)(*this)._currentPtr(4, 5). What is the proper way of calling pointer to method inside same class ?
我有这个使用指向成员函数的指针的示例代码，我想在运行时更改它，但我无法让它工作。我已经试过了this->*_currentPtr(4,5)(*this)._currentPtr(4, 5)。在同一个类中调用指向方法的指针的正确方法是什么？
The error : expression must have (pointer-to-) function type
错误：表达式必须具有（指向）函数类型
#include <iostream>
#include <cstdlib>
class A {
public:
void setPtr(int v);
void useFoo();
private:
typedef int (A::*fooPtr)(int a, int b);
fooPtr _currentPtr;
int foo1(int a, int b);
int foo2(int a, int b);
};
void A::setPtr(int v){
if(v == 1){
_currentPtr = foo1;
} else {
_currentPtr = foo2;
}
}
void A::useFoo(){
//std::cout << this->*_currentPtr(4,5); // ERROR
}
int A::foo1(int a, int b){
return a - b;
}
int A::foo2(int a, int b){
return a + b;
}
int main(){
A obj;
obj.setPtr(1);
obj.useFoo();
return 0;
}
回答by jrok
You need to tell the compiler which class the foos are coming from (otherwise it thinks they're functions from global scope):
您需要告诉编译器foos 来自哪个类（否则它认为它们是全局范围内的函数）：
void A::setPtr(int v){
if(v == 1){
_currentPtr = &A::foo1;
// ^^^^
} else {
_currentPtr = &A::foo2;
// ^^^^
}
}
and you need a set of parentheses here:
你需要一组括号：
std::cout << (this->*_currentPtr)(4,5);
// ^ ^
