Re: Best practice for passing std::string to functions?

"Igor Tandetnik" <>
Tue, 12 Jun 2007 08:36:01 -0400
"Norbert Unterberg" <nunterberg@newsgroups.nospam> wrote in message

What is the best practice to pass string data to a function? Is it a
better practice to pass the string by value or by const reference?

By const reference. Some experts (Herb Sutter, IIRC) would make an
exception for the following case:

// Taking a string and immediately making a copy
string f(const string& s) {
    string ret = s;
    // modify ret in some way
    return ret;

// Allegedly better:
string f(string s) {
    // modify s in some way
    return s;

The claim is that some compilers can perform a return value optimization
(RVO) on the second version but not the first. Personally, I don't like
the way an implementation detail leaks into interface in the second
case. The programmer using such function would have to stop and think
why it takes a string by value while everything else takes it by const

I have heared people saying that std::string is an optimized class
that has cheap copy operators

Some implementations of std::string use copy-on-write (COW), where two
strings that are copies of each other share the underlying data until
one of the strings is modified, at which time it gets its own copy.
However, such an implementation is next to impossible to make
thread-safe (and when you do make it thread-safe, you invoke
considerable overhead even for single-threaded programs). Most modern
STL implementations don't use COW. Thus it would be a bad idea to assume
COW is used.

Another popular technique is small string optimization (SSO), where
short enough strings are stored inside the string object itself,
avoiding heap allocations. With SSO, copying a short string (16
characters is a commonly used threshold) is cheap, but copying a longer
string is expensive as it involves a heap allocation.
With best wishes,
    Igor Tandetnik

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