CODEWITHSUNDEEP
c++expressionlanguage-lawyer
yggdrasil
yggdrasil

Reputation: 757

When is the type (not category) of an expression used?

I know every expression in c++ has a category (prvalue, xvalue, lvalue..) and a type which according to the standard draft, is never of reference type (may be cv qualified if not a prvalue)

5 If an expression initially has the type “reference to T” (8.3.2, 8.5.3), the type is adjusted to T prior to any further analysis. The expression designates the object or function denoted by the reference, and the expression is an lvalue or an xvalue, depending on the expression.

6 If a prvalue initially has the type “cv T,” where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.

Given that decltype has its own set of rules and the auto deduction has a different one as well, when does this “non-reference expression type” matter?

Upvotes: 2

Views: 141

Answers (1)

YSC
YSC

Reputation: 40150

It matters in unevaluated expressions:

typeid:

typeid(std::cout << 0) == typeid(std::ostream);
// true

noexcept:

template<class T> void f() noexcept(noexcept(T{}+T{}))

sizeof (even though sizeof has a specific rule, non contradicting with the rule for types of full expressions):

sizeof(std::cout << 0);
// the expression returns an std::ostream&, but its type is std::ostream

etc.

Upvotes: 1

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