There is no reason to add a const qualifier to the return type of a function that returns by value.
At best, it will be superfluous. At worst, it will be a performance bug: it can prevent move operation (when copy elision doesn’t take place). When
an object is const-qualified the copy constructor/assignment will be a better match than the move constructor/assignment.
One might think about adding this qualifier in order to forbid the call of unintended functions on the returned object. A common example is to
avoid unintended assignments:
X x1, x2, x3;
if (x1 + x2 = x3) { // Compiler will complain since const object cannot be assigned. Should be "x1 + x2 == x3"
...
}
C++11 introduced reference qualifiers for member functions. This feature provides a better approach to forbid calling unintended functions:
struct X {
X& operator=(const X& other) &;
};
...
X x1, x2, x3;
if (x1 + x2 = x3) { // Compiler will complain since assignment cannot be called on r-value. Should be "x1 + x2 == x3"
...
}
Noncompliant code example
class A {...};
const A f(); // Noncompliant
Compliant solution
class A {...};
A f();
