Jose Villalta Jose Villalta - 4 months ago 24
C Question

Are parentheses around the result significant in a return statement?

Is there a difference between these two statements inside a function?

bool returnValue = true;

//Code that does something

return(returnValue);


and this?

bool returnValue = true;

//Code

return returnValue;


The former has parentheses around
returnValue
.

Answer

C++14 adds a fringe case where parentheses around a return value may alter the semantics. This code snippet shows two functions being declared. The only difference is parentheses around the return value.

int var1 = 42;
decltype(auto) func1() { return var1; } // return type is int, same as decltype(var1)
decltype(auto) func1() { return(var1); } // return type is int&, same as decltype((var1))

In the first func1 returns an int and in the second one func1 returns an int& . The difference in semantics is directly related to the surrounding parentheses.

The auto specifier in its latest form was introduced in C++11. In the C++ Language Spec it is described as:

Specifies that the type of the variable that is being declared will be automatically deduced from its initializer. For functions, specifies that the return type is a trailing return type or will be deduced from its return statements (since C++14)

As well C++11 introduced the decltype specifier which is described in the C++ Language Spec:

Inspects the declared type of an entity or queries the return type of an expression. [snip] 1. If the argument is either the unparenthesised name of an object/function, or is a member access expression (object.member or pointer->member), then the decltype specifies the declared type of the entity specified by this expression.

  1. If the argument is any other expression of type T, then

    a) if the value category of expression is xvalue, then the decltype specifies T&&

    b) if the value category of expression is lvalue, then the decltype specifies T&

    c) otherwise, decltype specifies T

[snip]

Note that if the name of an object is parenthesised, it becomes an lvalue expression, thus decltype(arg) and decltype((arg)) are often different types.

In C++14 the ability to use decltype(auto) was allowed for function return types. The original examples are where the semantic difference with parentheses comes into play. Revisiting the original examples:

int var1 = 42;
decltype(auto) func1() { return var1; } // return type is int, same as decltype(var1)
decltype(auto) func1() { return(var1); } // return type is int&, same as decltype((var1))

decltype(auto) allows the trailing return type in the function to be deduced from the entity/expression on the return statement. In the first version return var1; is effectively the same as returning the type decltype(var1) (an int return type by rule 1 above) and in the second case return (var1); it's effectively the same as decltype((var1)) (an int & return type by rule 2b).

The parentheses make the return type int& instead of int, thus a change in semantics. Moral of the story - "Not all parentheses on a return type are created equal"