如何只對 int [3] 類型的指針使用“new”？

Question

我知道我們可以使用“new”來分配一個 object 或者分配一個數組。 那么我如何為如下所示的 int 數組新建一個 object 呢？ 我不能像新的數組一樣考慮新的整數。

也許int (*p)[3] = new int[1][3]；也和分配一個數組一樣。 但我想知道是否有另一種方法。

Answer 1

我不太明白你想問什么，但在cppreference中，有

new 表達式返回一個指向構造的 object 的純右值指針，或者，如果構造了對象數組，則返回“指向數組初始元素的指針”。

因此， new int[1][3]返回一個指向數組初始元素的指針，這意味着類型是int[3] 。 所以你可以用它來初始化指針int(*p)[3] ，它的類型是int(*)[3]

我想它可能會對你有所幫助。

Answer 2

正如退休的忍者所說，使用結構，如果可以的話，也盡量避免新建/刪除。 （特別是對於您將在編譯時知道其大小的數組）。 這是一個例子：

#include <array>
#include <iostream>
#include <memory>


// Always try to model things that have a name
// in the real world. Seperates the "How" (implementation details, in your case an array of values)
// from the "What" (what does your code represent)
// and reading code that has a 
struct point_t
{
    // initialize all values to defaults
    int x{};
    int y{};
    int z{};
};

// again the array is HOW you would implement it
// but what you are doing is defining a matrix
// so give it a name/type
struct matrix_t
{
    point_t values[3][3];
};

// you don't have to use pointers to pass matrices
// around by copying if you use references
// The const is there because show_matrix should
// NOT be able to modify the content of the matrix
// https://www.learncpp.com/cpp-tutorial/references/
void show_matrix(const matrix_t& matrix)
{
    // use range based for loops (unless you need the indices explicitly)
    // range based for loops can't go out of bounds
    // loop over the rows
    // https://en.cppreference.com/w/cpp/language/range-for
    for (const auto& row : matrix.values)
    {
        bool comma = false;

        // then loop over the values in the row
        for (const auto& point : row)
        {
            if (comma) std::cout << ", ";

            // normally I would overload operator<< for point_t
            std::cout << "(" << point.x << ", " << point.y << ", " << point.z << ")"; 
            comma = true;
        }
        std::cout << "\n";
    }
}


int main()
{
    // in c++ avoid new/delete if you can. (they're a main cause for bugs:
    // memory leaks, use after free, unitialized pointers (access violations) etc.)
    // specially if you now the sizes at compile time.
    // 
    // Read the following document 
    // https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines 
    // and look for new/delete and pointers
    matrix_t points;
    show_matrix(points);

    // if you really need to allocate dynamically
    // then use std::make_unique instead of new/delete
    // https://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique
    auto matrix_ptr = std::make_unique<matrix_t>();
    show_matrix(*matrix_ptr);

    return 0;
}