C++: Why is 'operator+=' defined but not 'operator+' for strings?

How come operator+= is defined for std::string but operator+ is not defined? See my MWE below (http://ideone.com/OWQsJk).

#include 

#include 
using namespace std;

int main() {  
    string first;
    first = "Day";
    first += "number";
    cout << "\nfirst = " << first << endl;

    string second;

    //second = "abc" + "def";       // This won't compile
    cout << "\nsecond = " << second << endl;
    return 0;
}

Answer

You need to convert one of the raw string literals to std::string explicitly. You can do it like others already mentioned:

second = std::string("abc") + "def";

or with C++14, you will be able to use

using namespace std::literals;
second = "abc"s + "def";
// note       ^