Allocation as default initialization

This code is not correct.

int* t = new int[l]; will happen before l=length;, thus reading the uninitialized variable l. Member initializers are handled before the constructor's body runs.

array::array(int length) : l{length} {}

instead would work because l is declared before t.

However, doing this "by hand" is a bad idea to begin with. You should be using std::vector.

The 2nd code snippet might have undefined behavior.

The data members are initialized at the order of how they're declared. For class array, when t is initialized l is not initialized yet. For objects with automatic and dynamic storage duration l will be initialized to indeterminate value, then the usage of l (i.e. new int[l]) leads to UB.

Note that l=length; inside the body of the constructor is just assignment; the initialization of data members has been finished before that.

BTW: With member initializer list the 1st code snippet chould be rewritten as

array::array(int length) : l(length), t(new int[l]) {
}