Compile error with non-constant array declaration, C++

Question

Weird, very weird case. Consider the code:

int n = 50;
auto p1 = new double[n][5]; //OK
auto p2 = new double[5][n]; //Error

main.cpp: In function 'int main()':
main.cpp:17:26: error: array size in new-expression must be constant
auto p2 = new double[5][n]; //Error

main.cpp:17:26: error: the value of 'n' is not usable in a constant expression
main.cpp:15:8: note: 'int n' is not const

Can anyone explain why do I get a compile error on the second one but the first one works perfectly?

Answer 1

With new double[n][5] you are allocating n values of type double[5] .

With new double[5][n] you are allocating 5 variable-length arrays . And C++ doesn't have VLA's so that's invalid.

The solution, as ever, is to use std::vector :

std::vector<std::vector<double>> p2(5, std::vector<double>(n));

The above defines p2 to be a vector of vectors of double . It constructs p2 to have a size of 5 elements, where each element is initialized to a vector of n values.

Answer 2

Your problem is explained, incidentially with the exact example that you give (funny!?) on the new[] expression page on cppreference under the section "Explanation". See excerpt:

If type is an array type, all dimensions other than the first must be specified as positive integral constant expression (until C++14) converted constant expression of type std::size_t (since C++14), but the first dimension may be any expression convertible to std::size_t. This is the only way to directly create an array with size defined at runtime, such arrays are often referred to as dynamic arrays.