Return string vs Passing string by reference to update the value

Question

What will be good programming practice out of two below funcs:

1. This:

    std::string buildvalue(const std::string &in) { std::string out; out = // Do some calulation bases on input return out; } 

2. Or this:

    void buildvalue(const std::string &in, std::string &out) { out = // Do some calulation bases on input } 

Cautious with 2 function is that the caller may pass non-empty string. Is there any points to note.

Answer 1

In first case, compiler will be able to optimize return value. It will place return value in calling function. For example,

std::string foo(...) { ... }

// ...

std::string result = foo(...);

Compiler will place foo return value right on the result spot. It relieves us from reference argument and premature variable declaring. Slightly C++17: Instead const std::string& you can use std::string_view. Its advantage is optional for creating temp std::string in following case:

void foo(const std::string&);
// ...
foo("hello world"); // here will be create std::string object

With std::string_view (c++17)

void foo(std::string_view);
// ...
foo("hello world"); // more productive

+ std::string have operator std::string_view, that leads its to std::string_view