c++ - <unresolved overloaded function type>

Question

In my class called Mat, I want to have a function which takes another function as a parameter. Right now I have the 4 functions below, but I get an error in when calling print(). The second line gives me an error, but I don't understand why, since the first one works. The only difference is function f is not a member of the class Mat, but f2 is. The failure is: 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
}

Why does this happen?

Solution

The problem here is that f2 is a method on Mat, while f is just a free function. You can't call f2 by itself, it needs an instance of Mat to call it on. The easiest way around this might be:

printf("%d\n", test([=](int v){return this->f2(v);}, 5));

The = there will capture this, which is what you need to call f2.

Comments

[2to1mux]

Since f2 and print are both member functions of Mat, isn't print allowed to call f2 without referencing a Mat object?

[Luchian Grigore]

Wow, this works (with a few tweaks from the original code). +1 from me

[Johannes Schaub - litb]

-1 . the error message says that the error happens because of an overloaded function, and not because the function template's function call syntax is wrong (which is what you say).

[2to1mux]

@JohannesSchaub-litb That isn't what he is saying. He's saying that f2 can only be called via an instance of a Mat object. This solution solves the issue (Although, I actually think it solves the original compiler error for a different reason than the one stated by Barry).

[Barry]

I think in this case the compiler error is not the best. f2 just isn't a function, &Mat::f2 is. If you tried to pass in that instead, you'd get a compile error about invalid call syntax.

[Admin]

This also gives me an error: "no matching function for call to 'Mat::test(Mat::print()::< lambda(int)>, int)'"

[fritzelr]

Minor nit: according to cppreference.com: The current object (*this) can be implicitly captured if either capture default is present. If implicitly captured, it is always captured by reference, even if the capture default is =. [[The implicit capture of *this when the capture default is = is deprecated. (since C++20)]]. So the capture should use [&].

[Jose Areas]

the question is about using functions as another function input. The issue you had is because you were passing a function instead of its return as a param to myFunction - value.x - "x" is a property of object "value". Which in your case, it is a method - value.x() - this is a call to the method "x"