如何在使用模板时初始化数组 <typename T> C ++

Question

Normally if I for example have string array[10] I can initialize all spots in the array like:

for(int i = 0; i < 10; i++)
    array[i] = "";

or if it is an array with pointers I write array[i] = nullptr instead, but how do I initialize when the type is more general like T array[10] ?

Answer 1

If value-initialization is all you need, then you can value-initialize all array members with empty braces:

T array[10]{};
//         ^^

Or:

T array[10] = {};

Value-initialization produces the zero or null value for scalars, value-initializes each member for aggregates, and calls the default constructor for non-aggregate class types.

(If you want to initialize your array elements with values other than T{} , then you need something more complex and specific.)

Answer 2

Well you could do like the standard does and use an initialization function in the form of

template<typename T, std::size_t N>
void initialize(T (&arr)[N], const T& value = T())
{
    for(auto& e : arr)
        e = value;
}

Now you can call this and not pass a value and the array with be initialized with default value or you can specify a value and the array will be initialized with that value.

Answer 3

If your goal is to call the default constructor on all elements of the array, you can use value-initialization:

#include <iostream>
#include <string>
#include <cassert>

template <typename T>
struct A {
  // value-initialize `array` which value-initializes all elements of `array`.
  A() : array() {}
  T array[10];
};

int main() {
    A<int> ints;

    for(auto e: ints.array) {
        assert(e == 0);
    }

    A<std::string> strings;

    for(auto e: strings.array) {
        assert(e.empty());
    }

    A<int*> int_ptrs;

    for(auto e: int_ptrs.array) {
        assert(e == nullptr);
    }

    // etc.
}