[c++] Concatenating strings doesn't work as expected

I know it is a common issue, but looking for references and other material I don't find a clear answer to this question.

Consider the following code:

#include <string>

// ...
// in a method
std::string a = "Hello ";
std::string b = "World";
std::string c = a + b;

The compiler tells me it cannot find an overloaded operator for char[dim].

Does it mean that in the string there is not a + operator?

But in several examples there is a situation like this one. If this is not the correct way to concat more strings, what is the best way?

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

I would do this:

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

Which compiles in VS2008.

std::string a = "Hello ";
std::string b = "World ";
std::string c = a;
c.append(b);