storage of user, error, exception messages (c++)

Question

Hello everyone.

Rather simple question. Where should I store error,exception, user messages? By far, I always declared local strings inside the function where it is going to be invoked and did not bother. e.g.

SomeClass::function1(...)
{
std::string str1("message1");
std::string str2("message2");
std::string str3("message3");
...
// some code
...
}

Suddenly I realized that since construction & initialization are called each time and it might be quite expensive. Would it be better to store them as static strings in class or even in a separate module? Localization is not the case here.

Thanks in advance.

Answer 1

Why not just use a string constant when you need it?

SomeClass::function1(...)
{
/* ... */
    throw std::runtime_error("The foo blortched the baz!");
/* ... */
}

Alternately, you can use static const std::strings. This is appropriate if you expect to copy them to a lot of other std::strings, and your C++ implementation does copy-on-write:

SomeClass::function1(...)
{
    static const std::string str_quux("quux"); // initialized once, at program start
    xyz.someMember = str_quux; // might not require an allocation+copy
}

If you expect to make lots of copies of these strings, and you don't have copy-on-write (or can't rely on it being present), you might want to look into using boost::flyweight.

Answer 2

TBH its probably best to ONLY construct error messages when they are needed (ie if something goes badly wrong who cares if you get a slowdown). If the messages are always going to appear then its probably best to define them statically to avoid the fact that they will be initialised each time. Generally, though, I only display user messages in debug mode so its quite easy to not show them if you are trying to do a performance build. I then only construct them when they are needed.