Question

I have a struct like this:

struct VrtxPros{
    long idx;
    std::vector<std::string> pros;
    VrtxPros(const long& _idx=-1, const std::string& val="") : idx(_idx)
    {
         if ( !val.empty() && val!="" )
             pros.push_back(val);
    }
};

and later in the code I use it like that:

long idx = 1234;
VrtxPros vp( 2134, std::string("-1") );
if ( margin ) vp.pros[0] = idx;

The compiler has no problem with that. I am wondering because the operator should deliver a reference.
I could not find an operator= in std::string which would accept a long as source.

Why does the code compile?

Answer 1

A std::string can be assigned to a char, and a long can be implicitly converted to a char, so a std::string can be assigned to a long. Your compiler will probably give a warning about this kind of implicit conversion (turn up the warning level and you’ll see it, if you don’t already).

See the #4 operator= listed here. Notice no constructor overload takes just a char, so this sort of thing can only be done for assignment.

For that matter, you can do this too:

std::string wow;
wow = 7ull; // implicit unsigned long long to char conversion
wow = 1.3f; // implicit float to char conversion