c ++ - <未解析的重载函数类型> [英] c++ - <unresolved overloaded function type>

问题描述

在我的类Mat中，我想有一个函数，它接受另一个函数作为参数。现在我有这4个函数，但是我在调​​用print（）时遇到错误。第二行给我一个错误，但我不明白为什么，如果第一个工作。唯一的区别是函数f不是类Mat的成员，而是f2。
它说：错误：没有匹配的函数调用'Mat :: test（<未解决的重载函数类型>，int）'

In my class called Mat, I want to have a function, which takes another function as a parameter. Right now I got these 4 functions, but I get an error in when calling print(). The second line gives me an error, but I don't understand why, if the first one works. The only difference is function f is not a member of the class Mat, but f2 is. It says: error: no matching function for call to 'Mat::test( < unresolved overloaded function type>, int)'"

template <typename F>
int Mat::test(F f, int v){
    return f(v);
}

int Mat::f2(int x){
    return x*x;
}

int f(int x){
    return x*x;
}

void Mat::print(){
    printf("%d\n",test(f ,5));    // works
    printf("%d\n",test(f2 ,5));    // does not work
}

为什么会发生这种情况？

Why does this happen?

推荐答案

指针的类型-member-function 不同于指针到函数。

函数的不同取决于它是一个普通函数还是某个类的非静态成员函数：

The type of a function is different depending on whether it is an ordinary function or a non-static member function of some class:

int f(int x);
the type is "int (*)(int)" // since it is an ordinary function

和

int Mat::f2(int x);
the type is "int (Mat::*)(int)" // since it is a non-static member function of class Mat

注意：如果它是类Fred的静态成员函数，它的类型就像是一个普通函数一样：int（*） ，float）

Note: if it's a static member function of class Fred, its type is the same as if it were an ordinary function: "int (*)(char,float)"

在C ++中，成员函数有一个隐式参数指向
对象（成员函数内的this指针）。正常C
函数可以被认为具有来自成员函数的不同的调用约定
，因此它们的指针的类型
（指针到成员函数vs指针到函数）不同，
不兼容。 C ++引入了一种新类型的指针，称为
指向成员的指针，只能通过提供对象来调用。

In C++, member functions have an implicit parameter which points to the object (the this pointer inside the member function). Normal C functions can be thought of as having a different calling convention from member functions, so the types of their pointers (pointer-to-member-function vs pointer-to-function) are different and incompatible. C++ introduces a new type of pointer, called a pointer-to-member, which can be invoked only by providing an object.

注意：不要尝试将指针成员函数转换为
指针函数;结果是不确定的，可能是灾难性的。
例如，指向成员函数的指针不需要包含相应函数的
机器地址。如最后一个
示例中所述，如果你有一个指向常规C函数的指针，使用
顶级（非成员）函数或静态（类）成员函数。

NOTE: do not attempt to "cast" a pointer-to-member-function into a pointer-to-function; the result is undefined and probably disastrous. E.g., a pointer-to-member-function is not required to contain the machine address of the appropriate function. As was said in the last example, if you have a pointer to a regular C function, use either a top-level (non-member) function, or a static (class) member function.

有关此此处和< a href =http://www.parashift.com/c++-faq-lite/addr-of-memfn.html>此处。

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