What differences, if any, between C++03 and C++11 can be detected at run-time?

Question

It is possible to write a function, which, when compiled with a C compiler will return 0, and when compiled with a C++ compiler, will return 1 (the trivial sulution with #ifdef __cplusplus is not interesting).

For example:

int isCPP()
{
    return sizeof(char) == sizeof 'c';
}

Of course, the above will work only if sizeof (char) isn't the same as sizeof (int)

Another, more portable solution is something like this:

int isCPP()
{
    typedef int T;
    {
       struct T 
       {
           int a[2];
       };
       return sizeof(T) == sizeof(struct T);
    }
}

I am not sure if the examples are 100% correct, but you get the idea. I believe there are other ways to write the same function too.

What differences, if any, between C++03 and C++11 can be detected at run-time? In other words, is it possible to write a similar function which would return a boolean value indicating whether it is compiled by a conforming C++03 compiler or a C++11 compiler?

bool isCpp11()
{ 
    //???
} 

Answer 1

Core Language

Accessing an enumerator using :::

template<int> struct int_ { };

template<typename T> bool isCpp0xImpl(int_<T::X>*) { return true; }
template<typename T> bool isCpp0xImpl(...) { return false; }

enum A { X };
bool isCpp0x() {
  return isCpp0xImpl<A>(0);
}

You can also abuse the new keywords

struct a { };
struct b { a a1, a2; };

struct c : a {
  static b constexpr (a());
};

bool isCpp0x() {
  return (sizeof c::a()) == sizeof(b);
}

Also, the fact that string literals do not anymore convert to char*

bool isCpp0xImpl(...) { return true; }
bool isCpp0xImpl(char*) { return false; }

bool isCpp0x() { return isCpp0xImpl(""); }

I don't know how likely you are to have this working on a real implementation though. One that exploits auto

struct x { x(int z = 0):z(z) { } int z; } y(1);

bool isCpp0x() {
  auto x(y);
  return (y.z == 1);
}

The following is based on the fact that operator int&& is a conversion function to int&& in C++0x, and a conversion to int followed by logical-and in C++03

struct Y { bool x1, x2; };

struct A {
  operator int();
  template<typename T> operator T();
  bool operator+();
} a;

Y operator+(bool, A);

bool isCpp0x() {
  return sizeof(&A::operator int&& +a) == sizeof(Y);
}

That test-case doesn't work for C++0x in GCC (looks like a bug) and doesn't work in C++03 mode for clang. A clang PR has been filed.

The modified treatment of injected class names of templates in C++11:

template<typename T>
bool g(long) { return false; }

template<template<typename> class>
bool g(int) { return true; }

template<typename T>
struct A {
  static bool doIt() {
    return g<A>(0);
  }
};

bool isCpp0x() {
  return A<void>::doIt();
}

---

A couple of "detect whether this is C++03 or C++0x" can be used to demonstrate breaking changes. The following is a tweaked testcase, which initially was used to demonstrate such a change, but now is used to test for C++0x or C++03.

struct X { };
struct Y { X x1, x2; };

struct A { static X B(int); };
typedef A B;

struct C : A {
  using ::B::B; // (inheriting constructor in c++0x)
  static Y B(...);
};

bool isCpp0x() { return (sizeof C::B(0)) == sizeof(Y); }

---

Standard Library

Detecting the lack of operator void* in C++0x' std::basic_ios

struct E { E(std::ostream &) { } };

template<typename T>
bool isCpp0xImpl(E, T) { return true; }
bool isCpp0xImpl(void*, int) { return false; }

bool isCpp0x() {
  return isCpp0xImpl(std::cout, 0);
}

Answer 2

I got an inspiration from What breaking changes are introduced in C++11?:

#define u8 "abc"

bool isCpp0x() {
   const std::string s = u8"def"; // Previously "abcdef", now "def"
   return s == "def";
}

This is based on the new string literals that take precedence over macro expansion.

Answer 3

How about a check using the new rules for >> closing templates:

#include <iostream>

const unsigned reallyIsCpp0x=1;
const unsigned isNotCpp0x=0;

template<unsigned>
struct isCpp0xImpl2
{
    typedef unsigned isNotCpp0x;
};

template<typename>
struct isCpp0xImpl
{
    static unsigned const reallyIsCpp0x=0x8000;
    static unsigned const isNotCpp0x=0;
};

bool isCpp0x() {
    unsigned const dummy=0x8000;
    return isCpp0xImpl<isCpp0xImpl2<dummy>>::reallyIsCpp0x > ::isNotCpp0x>::isNotCpp0x;
}

int main()
{
    std::cout<<isCpp0x()<<std::endl;
}

Alternatively a quick check for std::move:

struct any
{
    template<typename T>
    any(T const&)
    {}
};

int move(any)
{
    return 42;
}

bool is_int(int const&)
{
    return true;
}

bool is_int(any)
{
    return false;
}


bool isCpp0x() {
    std::vector<int> v;
    return !is_int(move(v));
}

Answer 4

Unlike prior C++, C++0x allows reference types to be created from reference types if that base reference type is introduced through, for example, a template parameter:

template <class T> bool func(T&) {return true; } 
template <class T> bool func(...){return false;} 

bool isCpp0x() 
{
    int v = 1;
    return func<int&>(v); 
}

Perfect forwarding comes at the price of breaking backwards compatibility, unfortunately.

Another test could be based on now-allowed local types as template arguments:

template <class T> bool cpp0X(T)  {return true;} //cannot be called with local types in C++03
                   bool cpp0X(...){return false;}

bool isCpp0x() 
{
   struct local {} var;
   return cpp0X(var);
}

Answer 5

This isn't quite a correct example, but it's an interesting example that can distinguish C vs. C++0x (it's invalid C++03 though):

 int IsCxx03()
 {
   auto x = (int *)0;
   return ((int)(x+1) != 1);
}

Answer 6

From this question:

struct T
{
    bool flag;
    T() : flag(false) {}
    T(const T&) : flag(true) {}
};

std::vector<T> test(1);
bool is_cpp0x = !test[0].flag;

Answer 7

Though not so concise... In current C++, class template name itself is interpreted as a type name (not a template name) in that class template's scope. On the other hand, class template name can be used as a template name in C++0x(N3290 14.6.1/1).

template< template< class > class > char f( int );
template< class > char (&f(...))[2];

template< class > class A {
  char i[ sizeof f< A >(0) ];
};

bool isCpp0x() {
  return sizeof( A<int> ) == 1;
}

Answer 8

#include <utility>

template<typename T> void test(T t) { t.first = false; }

bool isCpp0x()
{
   bool b = true;
   test( std::make_pair<bool&>(b, 0) );
   return b;
}