c++ - 在结构外调用指向函数的指针

Question

我有一个结构，里面有一个指向同一结构函数的指针。现在我需要调用一个指向结构外部函数的指针。我给出了以下代码的示例:

#include <iostream>

struct test {
    void (test::*tp)(); // I need to call this pointer-to-function
    void t() {
        std::cout << "test\n";
    }
    void init() {
        tp = &test::t;
    }
    void print() {
        (this->*tp)();
    }
};
void (test::*tp)();

int main() {
    test t;
    t.init();
    t.print();
    (t.*tp)(); // segfault, I need to call it
    return 0;
}

Answer 1

(t.*tp)();正在尝试调用成员函数指针 tp在全局命名空间中定义为 void (test::*tp)(); ，请注意它实际上被初始化为空指针(通过 zero initialization ¹)，调用它会导致 UB ，一切皆有可能。

如果要调用数据成员tp的 t (即 t.tp )在对象上 t ，你应该把它改成

(t.*(t.tp))();
     ^
     |
     ---- object on which the member function pointed by tp is called

如果你想调用全局 tp ，你应该适本地初始化它，比如

void (test::*tp)() = &test::t;

那么你可以

(t.*tp)(); // invoke global tp on the object t

---

¹关于零初始化

Zero initialization is performed in the following situations:

1) For every named variable with static or thread-local storage duration that is not subject to constant initialization (since C++14), before any other initialization.