初始化一个指向c++结构的指针数组

Initializing an array of pointers to structs in C can be done using compound literals.

在C中初始化指向结构体的指针数组可以使用复合文字。

typedef struct {
int a;
int b;
} s;

In C:

在C:

s *ptrArray[] = {
    &(s){
        .a = 1,
        .b = 2
    },
    &(s){
        .a = 4,
        .b = 5
    }
};

How can this be done in C++?

如何在c++中实现这一点?

I have also seen the difference in initializing structs in C++ not using compound statements:

我还看到了在c++中不使用复合语句初始化结构的区别:

s s1 = { a: 7, b: 8 };

2 个解决方案

#1

First - initializing anything to the address of a temporary value seems extremely fishy, in C as well. Are you sure that's valid? Hmmm. Anyway, a C++ compiler will really not let you do that.

首先，在C中初始化任何到临时值的地址看起来非常可疑。你确定那是有效的吗?嗯。无论如何，c++编译器不会让你这么做。

As for the your designated (named-field) initialization C++ line - it's actually non-standard, it's a GNU C++ extension, and you can't rely on it.

至于您指定的(命名字段)初始化c++行——它实际上是非标准的，它是GNU c++扩展，您不能依赖它。

You could do this:

你可以这样做:

struct s { int a, b; };

int main() {
    s data[] = { { 1, 2 }, { 4, 5 } };
    // instead of ptrArray[i], use &(data[i])
}

This compiles just fine. But - a more C++'ish version of this code would be:

这个编译。但是，更接近于c++版本的代码应该是:

#include <array>

struct s { int a, b; };

int main() {
    std::array<s, 2> data { s{ 1, 2 }, s{ 4, 5 } };
    // instead of ptrArray[i], use &(data[i]),
    // or use iterators, or ranged for loops
}

Why would you want to use std::array? Here's one explanation of the benefits. Actually, you could do slightly better and repeat yourself less with:

为什么要使用std::array?这里有一个关于好处的解释。实际上，你可以做得稍微好一点，少重复自己的话:

int main() {
    auto data = make_array(s{ 1, 2 }, s{ 4, 5 });
    // instead of ptrArray[i], use &(data[i]),
    // or use iterators, or ranged for loops
}

The make_array function is taken from here; you also have std::experimental::make_array(), but that's not standardized yet.

make_array函数取自这里;您还有std::experimental: make_array()，但这还没有标准化。

If you want to add or remove elements from data at run-time, you might switch to using std::vector:

如果您想在运行时从数据中添加或删除元素，您可以切换到使用std::vector:

#include <vector>

struct s { int a, b; };

int main() {
    std::vector<s> data { s{ 1, 2 }, s{ 4, 5 } };
    // instead of ptrArray[i], use &(data[i]),
    // or use iterators, or ranged for loops
}

#2

The reason your initialize was failing is you were attempting to initialize the array of pointers to struct to the address of numeric literal constants. The same as:

初始化失败的原因是，您试图初始化指针的数组，以构造数值常量的地址。一样:

#define A 5
int b = &A;    /* NOT HAPPENING */

(you can't take the address of 5)

(你不能取5的地址)

You can solve your problem by simply initializing an array of s instead of an array of pointers to s, e.g.:

只需初始化一个s数组而不是一个指向s的指针数组，就可以解决问题，例如:

    s ptrarr[] = { {1, 2}, {4, 5} };

With that change, your array will initialize fine, e.g.

有了这个更改，您的数组将会很好地初始化，例如。

#include <iostream>

typedef struct {
    int a;
    int b;
} s;

int main (void) {

    s ptrarr[] = { {1, 2}, {4, 5} };
    int cnt = 0;

    for (auto& i : ptrarr)
        std::cout << "ptrarr[" << cnt++ << "] : " << i.a << ", " << i.b << "\n";

}

Example Use/Output

使用/输出示例

$ ./bin/ptrarrystruct
ptrarr[0] : 1, 2
ptrarr[1] : 4, 5

#1