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c++decltypelvaluervalue

C++ arrow type yields lvalue

According to the C++ Primer, C++ arrow operator yields an lvalue. Additionally decltype of an expression which yields an lvalue will result in a reference type. So why the following decltype does not result in a reference type.

struct MyStruct {
   string name
};
MyStruct s;
s.name = "aname";
MyStruct* p = &s;
decltype (p -> name) str = s.name; //type of str will be string and not &string although p -> name yields an lvalue
Solution

  • From decltype (which is from §7.1.6.2/4 [dcl.type.simple]):

    If the argument is an unparenthesized id-expression or an unparenthesized class member access, then decltype yields the type of the entity named by this expression.

    p->name falls in the above case, so the type of decltype(p->name) is the type of p->name which is std::string not std::string&.

    On the other hand, decltype((p->name)) is std::string& because (p->name) is an lvalue expression.