Why does map<string, string> accept ints as values?

Question

I've recently been jarred by this:

#include <map>
#include <string>
#include <iostream>

using namespace std;

int main() {
    map<string, string> strings;
    strings["key"] = 88;  // surprisingly compiles
    //map<string, string>::mapped_type s = 88;  // doesn't compile as expected

    cout << "Value under 'key': '" << strings["key"] << "'" << endl;

    return 0;
}

It prints 'X' which is 88 in ASCII.

Why does a string map accept ints as values? The documentation for map's operator[] says it returns mapped_type& which is string& in this case and it doesn't have an implicit conversion from int, does it?

Answer 1

This is because, as you say, operator[] returns a std::string&, which defines an operator= (char c). Your commented-out example does not call the assignment operator, it is copy-initialization, which will try to call explicit constructors of the class, of which there are no applicable ones in std::string.

Answer 2

To complete the picture, note that:

strings[88] = "key"; 

does not compile as there is no std::string constructor from char/int. For char, std::string only defines the assignment operator.