storing mem_fun in a standard container

Question

Is there a way to create a vector< mem_fun_t< ReturnType, MyClass > > ?

The error i'm seeing is:

error C2512: 'std::mem_fun1_t<_Result,_Ty,_Arg>' : no appropriate default constructor available

Answer 1

You certainly can create such a vector.

#include <vector>
#include <functional>
#include <iostream>

struct MyClass
{
    int a()  { return 1; }
    int b()  { return 2; }
};

int main()
{
    std::vector<std::mem_fun_t<int, MyClass> > vec;
    vec.push_back(std::mem_fun(&MyClass::a));
    vec.push_back(std::mem_fun(&MyClass::b));
    MyClass x;
    for (size_t i = 0; i != vec.size(); ++i) {
        std::cout << vec[i](&x) << '\n';
    }
}

If you are having problems, read the error message carefully. For example, std::mem_fun can return all sorts of wrappers, depending on what you pass to it.

Or indeed, switch to boost's or C++0x's function.

---

Edit: With this particular error message, I assume that you are doing something that invokes the default constructor for contained type (e.g resize or specifying the size with the vector's constructor). You can't use those functions.

Answer 2

I really can't see why it would not work, but it's actually a pretty ugly solution. Just take vector<function<ReturnType(MyClass*)>> and be without those issues present in C++03 binders.

Answer 3

mem_fun_t meets the requirements to be stored in a container (it is copy-constructible and assignable), so the answer is yes.

However, it isn't default-constructible or comparable, so there are some things you can't do with a container of them, including:

• Resizing, unless you provide a value to fill with
• Constructing with a non-zero size, unless you provide a value to fill with
• Comparing containers

The error you are seeing comes from trying to either resize, or construct with a size.