Is this string addition a valid expression?

Question

I was curious if the compound assignment operators are valid for multiple parameters. My guess is += will have no side-effect but may not be the same case with "-=".

std::string a; 
a += "Hello" + " "+"World"; // doesn't compile
std::string name = "Old";
a += "Hello" + name +"World"; // compiles

Answer 1

This is not a valid expression because there is no operator + for string literals

"Hello" + " "+"World

(more precisely for pointers because in expressions string literals with rare exceptions are converted to pointers to their first symbols.)

Instead you could write

std::string a; 
( ( a += "Hello" ) += " " ) += "World";

But it would be more readable if to write

a += "Hello";
a += " ";
a += "World";

Or as @Bathsheba pointed out in a (valuable) comment to my answer you could use a user-defined string literal the following way

#include <string>
#include <iostream>

int main()
{
    using namespace std::string_literals;
    std::string a; 
    a += "Hello"s + " " + "World";

    std::cout << a << '\n';
}

As for this statement

a += "Hello" + name +"World";

then it can be rewriten using the operators defined for the class std::basic_string

template<class charT, class traits, class Allocator>
basic_string<charT, traits, Allocator>
operator+(const charT* lhs, const basic_string<charT, traits, Allocator>& rhs);

and

template<class charT, class traits, class Allocator>
basic_string<charT, traits, Allocator>
operator+(basic_string<charT, traits, Allocator>&& lhs, const charT* rhs);

like

a += operator +( operator +( "Hello", name ), "World" ); 

For example

#include <string>
#include <iostream>

int main()
{
    std::string a;
    std::string name = "Old";

    a += operator +( operator +( "Hello ", name ), " World" ); 

    std::cout << a << '\n';
}

Take into account that each operator returns an object of the type std::basic_string for which the operator + is defined. That is in each call of the operators there is present an object of the type std::basic_string as an argument.