In C++ we have an operator::
, which allows us to do things like that:
typedef std::pair<A, B> pairAB;
pairAB p;
pairAB::first_type a(42);
pairAB::second_type b("hello world");
p.first = a;
p.second = b;
I tried to use this operator on the object of this type:
pairAB p2;
p2::first_type a2(42);
p2::second_type b2("hello again");
p2.first = a2;
p2.second = b2;
And the compiler showed me error: C2510: 'p2' : left of '::' must be a class/struct/union
, and I want to know is there any trick which allows me to do that?
If you are using GCC, there is a pre-C++11 equivalent called __typeof__
that behaves similarly. Try:
#include <string>
int main()
{
typedef std::pair<int, std::string> pairAB;
pairAB p2;
__typeof__(p2.first) a2 = 42;
__typeof__(p2.second) b2 = "hello again";
p2.first = a2;
p2.second = b2;
}
Alternatively, if you can use boost , it provides Boost.typeof which should work in older Visual C++ compilers as well.
Getting the type of a variable is exactly what decltype
keyword is for in c++11.
decltype(p2.first) a2 = 42;
Prior to c++11 there was no standard way to do that. There are compiler specific features such as gcc's typeof
and library based implementations such as Boost.Typeof that have existed prior to c++11 and do a similar thing.
There's no need for C++11 features nor compiler extensions like typeof
in C++03, which are not portable. If the variable is being used, its type is available somewhere -let's say it is T in a template or a function signature.
Then you can simply typename T::first_type
since pair
carries with itself member type information for its member types (as do many other STL classes such as containers, string
, ...).
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