Question

When I try to compile

template<int dim> 
struct Foo
{
    Foo(const int (&i)[dim]) {}
};

int main()
{
    Foo<2> f = Foo<2>((int[2]){0});  // line 9
    return 0;
}

I get the compilation error

test.cpp:9:31: error: no matching function for call to ‘Foo<2>::Foo(int [1])’

Apparently, the argument I pass to the constructor is regarded as an int[1]. Why isn’t it regarded as an int[2] (which could then be casted to a const reference as expected by the constructor)? Shouldn’t the missing elements be value-initialized according to 8.5.1 (7)?

After all, replacing line 9 with

int arg[2] = {0};
Foo<2> f = Foo<2>(arg);

lets me compile the program. Additionally, when I try to pass (const int [2]){0, 0, 0} to the constructor, I get the error message too many initializers for ‘const int [2]’, so apparently, the compiler is trying to construct a const int[2].

Somebody please shed some light on this unintuitive behavior.

Answer 1

The construct (int[2]){0} is a C99 compound literal, which is not part of C++. How particular compilers interpret in the context of C++ is anyone’s guess (or a matter of examining the source code).

PS. OK, it seems that gcc 4.7/gcc 4.8/clang-3.1 handle it quite sensibly – the type of the compound literal is the same as the C99 standard specifies it.

I guess the OP compiler is a bit older.