CODEWITHSUNDEEP
c++visual-c++stdc-preprocessor
Class Skeleton
Class Skeleton

Reputation: 3073

How to use std::is_same to generate compile time errors?

I am using a third party API, which includes a header file that contains a set of typedefs. Over the past 4 years there have been minor changes to some of the typedefs (e.g. switching between unsigned/signed, changing from int to long etc).

I want to add a compile time check in my code so that I know if a specific typedef has changed. I was thinking of adding something like the below:

#if !std::is_same<::ApiType, int>::value
#error Type has changed
#endif

When I tried this out across various typedefs I found that a compile error was always being thrown.

I set up a small console program, which showed the same problem (i.e. always false for preprocessor usage) but was fine outside of the preprocessor:

#include "stdafx.h"
#include <Windows.h>
#include <type_traits>

int main()
{
#if std::is_same<int, int>::value
  const auto aa = 14; // omitted
#else
  const auto bb = 17;
#endif

#if std::is_same<::DWORD, int>::value
  const auto cc = 14; // omitted
#else
  const auto dd = 17;
#endif

  const auto a = std::is_same<int, int>::value; // true
  const auto b = std::is_same<::DWORD, int>::value; // false
  const auto c = std::is_same<::DWORD, unsigned long>::value; // true

  return 0;
}

I am using Visual Studio 2015.

How do I implement such a compile time check on expected types (specifically to produce a compile time error if the types are not the same)?

Upvotes: 2

Views: 1347

Answers (1)

Biffen
Biffen

Reputation: 6355

The preprocessor doesn't know anything about types. (Hint: It runs before compilation, hence the ‘pre’.)

What you want is static_assert. E.g:

static_assert(std::is_same<::ApiType, int>::value,
              "Type has changed");

Although, since it's an assertion, perhaps it should say ‘has not’.

You can put that almost anywhere, even outside of any function.

Upvotes: 6

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