Statement to determine the type of an object in C++

Question

Is there a statement s_type which allows to determine the type of an object in C++?

E.g.

s_type(int a);      // int
s_type(&a);         // int*

``. There are ways to print the type, but prefer boost when it comes to that. — sweenish, May 20 '20 at 16:53
[There](https://en.cppreference.com/w/cpp/header/type_traits) you go... — kesarling He-Him, May 20 '20 at 16:56
Do you want to print the type, or do something else with it? — HolyBlackCat, May 20 '20 at 16:58
@ArdentCoder the compiler gives me the error: ‘type_name’ was not declared in this scope — Federica Guidotti, May 20 '20 at 17:10
@FedericaGuidotti I followed the top answer in that post and I got your expected results: [Live Demo](https://godbolt.org/z/4EsKhD). — Ardent Coder, May 20 '20 at 17:24

score 1 · Answer 1 · answered May 20 '20 at 16:56

1

It's called decltype:

int a = 0;
decltype(a) b = 42;

answered May 20 '20 at 16:56

bipll

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1

how can i print it? – Federica Guidotti May 20 '20 at 17:01
2

@FedericaGuidotti Types cannot be printed in C++. You can get something out of `typeid`. – eerorika May 20 '20 at 17:13
@eerorika can you show me an example for an int object please? – Federica Guidotti May 20 '20 at 17:18
@FedericaGuidotti: You cannot print a typename; whatever you might get out of `typeid` will be implementation-defined and is almost certainly not the human-readable C++ name of the type. At present, this is not a thing that's possible in C++. – Nicol Bolas May 20 '20 at 17:24
... without some extra effort. – bipll May 20 '20 at 18:02

score 1 · Accepted Answer · answered May 20 '20 at 17:20

#include <boost/type_index.hpp>
#include <iostream>

int main() {
  using boost::typeindex::type_id_with_cvr;
  using std::cout;

  int a = 42;
  int& b = a;
  int* c = &a;

  cout << type_id_with_cvr<decltype(a)>().pretty_name() << '\n';
  cout << type_id_with_cvr<decltype(b)>().pretty_name() << '\n';
  cout << type_id_with_cvr<decltype(c)>().pretty_name() << '\n';
}

Output is:

int  
int&  
int*

If I want to see the types, this is the method I use. It does require boost and some people don't like to see third-party libraries, but this does the job much better than C++'s typeid.

Statement to determine the type of an object in C++

2 Answers2