CODEWITHSUNDEEP
c++arrayspointersstructdesignated-initializer
vg34
vg34

Reputation: 91

Initializing an array of pointers to structs in C++

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

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

In C:

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

How can this be done in C++?

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

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

Upvotes: 3

Views: 2173

Answers (2)

einpoklum
einpoklum

Reputation: 132310

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.

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.

You could do this:

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:

#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:

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.

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

Upvotes: 2

David C. Rankin
David C. Rankin

Reputation: 84652

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:

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

(you can't take the address of 5)

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

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

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

#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

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

Upvotes: 1

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