Suggested speed improvement when defining string with value immediately, instead of delaying

Question

I'm currently reading in "The C++ Programming Language: Special Edition" by Bjarne Stroustrup and on page 133 it states the following:

For user-defined types, postponing the definition of a variable until a suitable initializer is available can also lead to better performance. For example:

string s;  /* .... */ s = "The best is the enemy of the good.";

can easily be much slower than

string s = "Voltaire";

I know it states can easily, which means it won't necessarily be so, however let's just say it does occur.

Why would this make a potential performance increase?

Is it only so with user-defined types (or even STL types) or is this also the case with int, float, etc?

Answer 1

I'd say this is mainly about types with non-trivial default constructors, at least as far as performance is concerned.

The difference between the two approaches is that:

• In the first version, an empty string is first constructed (using the default constructor); then the assignment operator is used to effectively throw away the work done by the default constructor, and to assign the new value to the string.
• In the second version, the required value is set right away, at the point of construction.

Of course, it is really hard to tell a priori how big a performance difference this would make.