Could not convert from '<brace-enclosed initializer list> to

Question

I know that has a lot of questions similar, but I saw them and none of them helped me, I think is that because mine is kind of different, and at the same time weird.

I made another question and a member answered to me, but instead of using classes he used structs. and it's working perfectly, but when I try to put it as classes, using the same logic, this is what happen, the error:

error: could not convert '{{"foo", "bar"}}' from '<brace-enclosed initializer list>' to 'B'

I tried, but I don't know what is happening.

#include <iostream>
#include <map>

class A
{
public:
    A() {}
    A(const std::string & input) : data(input) {}
private:
    std::string data;
};

class B
{
public:
    B();
    B(std::initializer_list<std::pair<std::string, A>> input) : container(begin(input), end(input)) {}
private:
    std::map<std::string, A> container;
};

int main( int argc, char ** argv )
{
    B b = {
        {"foo", "bar"}
    };

    return 0;
}

Also the answer of the member here: Ideone

Thank you all.

Solution

You must initialize the 'b' like this:

B b = {
    { "foo", A{"bar"} }
};

Because {"foo", "bar"} is of type {string, string} instead of {string, A}

Comments

[chris]

A string literal to an A is two user-defined conversions, so that's not going to work implicitly.

[SH.0x90]

Yes, you are right, I just tried this removing the "bar" and putting std::string("bar") and it worked. Do you have a solution to this work in both ways?

[chris]

You could make an overload of A's constructor that takes a const char * and delegates to the other one.

[Adam H. Peterson]

The only difference between struct and class is default access: public versus private. (In fact, the only time I ever use the class keyword anymore is with template-template parameters.) Do you mean aggregate versus non-aggregate?

[SH.0x90]

I prefer to use classes sometimes because of the encapsulation, and it's a thing that I REALLY care about. Nut there are situations that you can use for sure the structs without concern.

[Adam H. Peterson]

You can get the same encapsulation with struct you can with class. If the first thing you're going to do is put public: at the top, there's not really any effective difference.