Materialization

Materialization is the step by which prvalue is used to create a temporary object in memory. Temporary objects have existed since C++98, but the timing of prvalue materialization was redefined in C++17. Since C++17, prvalues do not “immediately” materialize temporary objects; materialization occurs only when a temporary object is required. In other words, C++17 defers the materialization of a prvalue “as long as possible”. [cf. class. temporary] This change guarantees copy elision.

For example:

class C{};
C create() { return C{}; }
void foo(C c){}
 
int main(){
 foo(create());
}

Behavior before C++17 without copy elision:

1. Create and initialize (materialize occurs here) a temporary object with prvalue C{}. (used default ctor)
2. Construct another return object from the temporary. (used move ctor)
3. The temporary object returned by create() is then moved into function parameter c.

Behavior since C++17:

1. Construct an object as the function parameter of foo() with the prvalue C{}(materialize happens here), and then pass the object into the caller.

Comparison

By comparing these two behaviors, we can see that the advantage of deferring the materialization of prvalues is that no copy or move constructor needs to be invoked; threrfore, copy elision is guaranteed.

Please see below for more details: How does a function return local object internally? Name Returned Value Optimization Avoid Unnecessary std::move()