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问题:
Unless I am mistaken, it should be possible to create a std:array in these ways:
std::array<:string> strings = { "a", "b" }; std::array<:string> strings({ "a", "b" });
And yet, using GCC 4.6.1 I am unable to get any of these to work. The compiler simply says:
expected primary-expression before ',' token
and yet initialization lists work just fine with std::vector. So which is it? Am I mistaken to think std::array should accept initialization lists, or has the GNU Standard C++ Library team goofed?
回答1:
std::array is funny. It is defined basically like this:
template struct std::array { T a[size]; };
It is a struct which contains an array. It does not have a constructor that takes an initializer list. But std::array is an aggregate by the rules of C++11, and therefore it can be created by aggregate initialization. To aggregate initialize the array inside the struct, you need a second set of curly braces:
std::array<:string> strings = {{ "a", "b" }};
Note that the standard does suggest that the extra braces can be elided in this case. So it likely is a GCC bug.
回答2:
To add to the accepted answer:
std::array a1{'a', 'b'}; std::array a2 = {'a', 'b'}; std::array a3{{'a', 'b'}}; std::array a4 = {{'a', 'b'}};
all work on GCC 4.6.3 (Xubuntu 12.01). However,
void f(std::array a) { } //f({'a', 'b'}); //doesn't compile f({{'a', 'b'}});
the above requires double braces to compile. The version with single braces results in the following error:
../src/main.cc: In function ‘int main(int, char**)’: ../src/main.cc:23:17: error: could not convert ‘{'a', 'b'}’ from ‘’ to ‘std::array’
I'm not sure what aspect of type inference/conversion makes things work this way, or if this is a quirk of GCC's implementation.
