Compile-time assertion?

Question

Is there a way I can assert that two constant expressions are equal at compile time?

e.g. I want this to cause a compile-time error

enum { foo=263, bar=264 };
SOME_EXPRESSION(foo,bar)

but I want this to not cause an error

enum { foo=263, bar=263 };
SOME_EXPRESSION(foo,bar)

---

edit: the above was simplified. My situation is more like

some_other_file_I_dont_control.h:

class X
{
public:
   enum { foo=263 };
}

my_file.h:

enum { bar=something+somethingelse }; // bar should equal X::foo
SOME_EXPRESSION(X::foo, bar)

Answer 1

Yes. You can do this with template specializations on type bool, like this:

// empty default template
template <bool b>
struct StaticAssert {};

// template specialized on true
template <>
struct StaticAssert<true>
{
    static void assert() {}
};

int f()
{
    StaticAssert<1==1>::assert();   // compiles fine, assert() member found
    StaticAssert<1==2>::assert();   // compile failure, no assert() member for StaticAssert<false>
}

Code is basically from memory, may need some tweaks.

Answer 2

See static_assert (C++0x only); if on an older version, see Boost's StaticAssert.

Answer 3

For another version of a static assert, that you can glorify by adding a better name, you can use:

// name must be a valid identifier
#define STATIC_ASSERT( condition, name )\
    typedef char assert_failed_ ## name [ (condition) ? 1 : -1 ];

And use as:

STATIC_ASSERT( x == y, constants_must_be_same );

The compiler will trigger an error similar to:

size of array 'assert_failed_constants_must_be_same' is negative

Which does not seem that helpful, but it will point to the exact line of the assert, and after a while you will start processing that error message as static assert failed

Answer 4

Another possibility for Windows is C_ASSERT, which is defined if Windows.h is included.

Answer 5

There is also the trick to use a switch (..) statement. Kind of old style though. The case entry foo == bar has to be compile time evaluated and if it happens to be false the switch statement will cause an error. The compiler will also reduce it to "nothing".

{ 
  bool x=false; 
  switch (x) {
  case foo == bar:
    break;
  case false:
    // Compile time test that foo == bar
    break;
}

Answer 6

you can define your own static assertion , this way :

#include <iostream>
template <bool b> class ClassStaticAssert;
template <>
class ClassStaticAssert<true>{static const bool value = true;};
#define STATIC_ASSERT(e) (ClassStaticAssert<e>())
int main()
{
    STATIC_ASSERT(0);
    return 0;
}

Answer 7

Similar to iammillind's solution, which was unfortunately one only useful at runtime:

template <int A, int B>
class VALUES { 
};

// specialization to provide safe passage for equal values        
template <int X>
class VALUES<X, X> {
public:
   static void MY_VALUES_ARE_EQUAL() {}
};


#define ASSERT_EQUALITY(a, b)    \
{    \
   typedef VALUES<a, b> COMPILE_TIME_ASSERTION;       \
   COMPILE_TIME_ASSERTION::VALUES_ARE_EQUAL();     \
}

int main() {
   ASSERT_EQUALITY(1, 1);    // compiles just fine
   ASSERT_EQUALITY(1, 2);    // ERROR!
   // . . . 
 }

The nice thing about this is that it provides a nice compiler message. My compiler tells me the following:

‘VALUES_ARE_EQUAL’ is not a member of ‘COMPILE_TIME_ASSERTION {aka VALUES<1, 2>}’

You don't need the typedef. Without:

'VALUES_ARE_EQUAL' is not a member of 'VALUES<1, 2>'

Of course, there are a bunch of other ways to generate helpful messages. For giggles:

// these give use some tips in the compiler warnings
class COMPILE_TIME_EQUALITY_ASSERTION {} compiler_message; 
class EQUAL_VALUES_ONLY_PLEASE {};


template <int A, int B>
class VALUES {
public:
   static void AreEqual(EQUAL_VALUES_ONLY_PLEASE) {}
};

template <int X>
class VALUES<X, X>
{
public:
   static void AreEqual(COMPILE_TIME_EQUALITY_ASSERTION) {}
};


#define ASSERT_EQUALITY(a, b)                                   \
{                                                               \
   VALUES<a, b>::AreEqual(compiler_message);                             \
}

int main() {
    ASSERT_EQUALITY(1, 1) // a-okay
    ASSERT_EQUALITY(1, 2) // ERROR!
}

I get the following compiler errors:

no matching function for call to:
‘VALUES<1,2>::AreEqual(COMPILE_TIME_EQUALITY_ASSERTION&)' 
candidate is:
static void VALUES<\A, B>::AreEqual(EQUAL_VALUES_ONLY_PLEASE) [with int A = 1, int B = 2]

combinations of static member functions/constructors/field assignment/privacy and template specifications can yield different results perhaps more appropriate for your situation.

Answer 8

template <int a, int b>
inline void static_assert_equal()
{
    typedef char enum_values_must_be_equal[a == b ? 1 : -1];
    (void) sizeof(enum_values_must_be_equal);
}

int main()
{
    enum { foo = 1, bar = 2, fum = foo };
    static_assert_equal<foo, fum>(); // compiles ok
    static_assert_equal<foo, bar>(); // fails at compile time
    return 0;
}

This derives from the checked_delete idiom.

Answer 9

I suggest to take a look at the Eigen library static assert mechanism:

http://eigen.tuxfamily.org/dox/StaticAssert_8h_source.html

Answer 10

You can do some preprocessor magic like

#define FOO_VALUE 263
#define BAR_VALUE 264

enum {foo=FOO_VALUE, bar=BAR_VALUE}

#if !(FOO_VALUE == BAR_VALUE)
#error "Not equal"
#endif

Answer 11

I would go for one of the available static_asserts.

• boost::static_assert
• C++0x static_assert

But just because I have never tried before I wrote this:

enum { foo=263, bar=264 };

template<bool test>
struct CompileAssert
{
    bool assert() {}
};

template<>
struct CompileAssert<false>  {}; // fail on false.

int main()
{
    CompileAssert<foo != bar>().assert();  // Now I have seen Chad above I like his static 
    CompileAssert<foo == bar>().assert();  // method better than using a normal method.
}                                          // But I tried zero length arrays first did 
                                           // not seem to work

Answer 12

template<int X, int Y>
struct Check
{
  enum { value = false };
};

template<int X>
struct Check<X,X>
{
  enum { value = true };
};

I have taken the example of int. You can change it according to your need. Here is the demo. Usage:

Check<foo, bar>::value