Another C++ learning moment: returning strings from functions

Question

I've got some basic questions about C++. Consider the following code in which I attempt to return a string.

const std::string&
NumberHolder::getValueString() {
    char valueCharArray[100];
    sprintf_s(valueCharArray,"%f",_value);
    std::string valueString(valueCharArray);
    return valueString;
}

I'm attempting to return a string with the value of a class member called _value. However I'm getting the warning that I'm trying to pass back a pointer to a local variable. This is of course a bad thing. If I understand C++ enough at this point, this means that the pointer I pass back will already have delete called on it by the time someone tries to use it. So I modify:

const std::string&
NumberHolder::getValueString() {
    char valueCharArray[100];
    sprintf_s(valueCharArray,"%f",_value);
    std::string valueString = new std::string(valueCharArray);
    return (*valueString);
}

This should create a pointer on the stack which will survive outside of this function. Two problems here though: 1) it doesn't compile anyway and I don't understand why (error = cannot convert from 'std::string *' to 'std::basic_string<_Elem,_Traits,_Ax>') and 2) This seems like a potential memory leak because I'm depending upon someone else to call delete on this guy. What pattern should I be using here?

Answer 1

You're defeating the point of having a std::string by allocating it on the heap!

Just return it by value like this:

std::string NumberHolder::getValueString()
{ 
    char valueCharArray[100]; 
    sprintf_s(valueCharArray,"%f",_value); 
    return std::string(valueCharArray); 
} 

Just about every compiler nowadays will do return value optimization (RVO) on the return statement, so no copies should be made. Consider the following:

NumberHolder holder;
// ...
std::string returnedString = holder.getValueString();

With RVO, the compiler will generate the code for the above implementation of NumberHolder::getValueString() such that std::string is constructed at the location of returnedString, so no copies are needed.