Display integer at compile time in static_assert()

Question

Here is a simplified version of what I'm trying to do

enum First
{
    a,
    b,
    c,
    nbElementFirstEnum,
};
enum Second
{
    a,
    b,
    c,
    nbElementSecondEnum,
};

static_assert(
    First::nbElementFirstEnum == Second::nbElementSecondEnum,
    "Not the same number of element in the enums.");
/*static_assert(  
    First::nbElementFirstEnum == Second::nbElementSecondEnum, 
    "Not the same number of element in the enums." + First::nbElementFirstEnum + " " + Second::nbElementSecondEnum);*/

But I would like to be able to print the value of First::nbElementFirstEnum and Second::nbElementSecondEnum in the assert message (like in the commented version which obviously doesn't work). I have tryed using macro concatenation with "#". I also tryed using variadic templates, retrieveing with %10 each number and adding the '0' character to the value retrieved, but all I get is a constexpr char[].

So my question is how can I get my enums values to be printed in a string literal.

Possible duplicates :

C++11 static_assert: Parameterized error messages

Integrate type name in static_assert output?

The most interesting topic was this one: Printing sizeof(T) at compile time But I don't want to have a warning, or decomment code to know the values.

Answer 1

This basically works, although it's possible to break with a little effort (by making V1 and V2 sum to a multiple of 256). So, I think your solution is uglier but still more robust.

template <int V1, int V2> struct AssertEquality
{
    static const char not_equal_warning = V1 + V2 + 256;
};

template <int V> struct AssertEquality<V, V>
{
    static const bool not_equal_warning = 0;
};

#define ASSERT_EQUALITY(V1, V2) static_assert( \
    AssertEquality<static_cast<int>(V1), \
                   static_cast<int>(V2)>::not_equal_warning == 0, \
    #V1 " != " #V2 );

// ...

ASSERT_EQUALITY(First::nbElementFirstEnum, Second::nbElementSecondEnum);

with output looking like:

g++ -std=c++0x -c chksz.cpp
chksz.cpp: In instantiation of ‘const char AssertEquality<3, 2>::not_equal_warning’:
chksz.cpp:40:124:   instantiated from here
chksz.cpp:5:53: warning: overflow in implicit constant conversion
chksz.cpp:40:1: error: static assertion failed: "First::nbElementFirstEnum != Second::nbElementSecondEnum"

---

For reference, this original version depended on gcc printing the static_assert message even when the boolean condition doesn't compile at all.

template <typename Enum1, int Max1, typename Enum2, int Max2>
struct AssertSameSizeEnums;

template <typename Enum1, int EnumMax, typename Enum2>
struct AssertSameSizeEnums<Enum1, EnumMax, Enum2, EnumMax> {};
// only define the special case where Max1 and Max2 have the same integer value

#define ASSERT_SAME_SIZE_ENUMS(E1, M1, E2, M2) static_assert( \
    sizeof(AssertSameSizeEnums<E1, E1::M1, E2, E2::M2>), \
    #E1 "::" #M1 " != " #E2 "::" #M2 );

enum class First {
    a, b, c, nbElementFirstEnum,
};
enum class Second {
    a, b, c, nbElementSecondEnum,
};

ASSERT_SAME_SIZE_ENUMS(First, nbElementFirstEnum, Second, nbElementSecondEnum);

Note I changed your enums to be strongly-typed, because otherwise the enumerated constant names clashed. If you have weakly-typed enums, the First and Second passed to the macro should name the enclosing scope.

Now, if I comment out one of the values (so the enums are different sizes), I get:

g++ -std=c++0x -c chksz.cpp
chksz.cpp:25:113: error: invalid application of ‘sizeof’ to incomplete type ‘AssertSameSizeEnums<First, 3, Second, 2>’
chksz.cpp:25:1: error: static assertion failed: "First::nbElementFirstEnum != Second::nbElementSecondEnum"

See how the integer values are displayed in the incomplete type error, and the symbolic names in the static assertion?