I would like to define the size of a multidimensional array as a constant. What would be the best way to do this please? In the example below, MY_CONSTANT would be equal to 2. Can this constant be determined before run time?
#define MY_CONSTANT <what goes here?>
std::string myArray[][3] = {
{"test", "test", "test"},
{"test", "test", "test"}
};
You might use std::extent to get the size of the array at compile time:
std::string myArray[][3] = {
{"test", "test", "test"},
{"test", "test", "test"}
};
const std::size_t myArraySize = std::extent<decltype(myArray)>::value;
The preprocessor will not be able to define the value directly. Of course you could use
#define MY_CONSTANT std::extent<decltype(myArray)>::value
but I guess that's not really what you want to do.
In C++11, you can use a constexpr function to initialise a compile-time constant:
template <typename T, size_t N>
constexpr size_t array_size(T(&)[N]) {return N;}
constexpr size_t MY_CONSTANT = array_size(myArray);
(or use std::extent as another answer suggests; I didn't know about that).
Historically, you would need to initialise it with a constant expression like
const size_t MY_CONSTANT = sizeof(myArray) / sizeof(myArray[0]);
you are using C++, better use const instead of #define.
#define is a preprocessor directive which will perform textual substitution before compilation.
const int create a read only variable.
so better use something like: const size_t arraySize = 2;
"Can this constant be determined before run time?"
you would have to use dynamically allocated arrays using new
or better use vector from STL
const std::size_t array_size = 2;? BTW, preferstd::arraywhen you have constant size array, elsestd::vector(orstd::deque).constvariables to#define. They can be used as compile-time constants, and also respect scope.