“” + something in C++

Question

I\'ve been having really freaky stuff happening in my code. I believe I have tracked it down to the part labeled \"here\" (code is simplified, of course):

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Answer 1

There are a lot of explanations of how the compiler interprets this code, but what you probably wanted to know is what you did wrong.

You appear to be expecting the + behavior from std::string. The problem is that neither of the operands actually is a std::string. C++ looks at the types of the operands, not the final type of the expression (here the return type, std::string) to resolve overloading. It won't pick std::string's version of + if it doesn't see a std::string.

If you have special behavior for an operator (either you wrote it, or got a library that provides it), that behavior only applies when at least one of the operands has class type (or reference to class type, and user-defined enumerations count too).

If you wrote

std::string("") + c

or

std::string() + c

or

""s + c // requires C++14

then you would get the std::string behavior of operator +.

(Note that none of these are actually good solutions, because they all make short-lived std::string instances that can be avoided with std::string(1, c))

The same thing goes for functions. Here's an example:

std::complex ipi = std::log(-1.0);

You'll get a runtime error, instead of the expected imaginary number. That's because the compiler has no clue that it should be using the complex logarithm here. Overloading looks only at the arguments, and the argument is a real number (type double, actually).

Operator overloads ARE functions and obey the same rules.