Elegant way to initialize array of structs in C++98

Question

I am using gcc version 4.9.2 If I compile using the compiler flag -std=c++0x the following code compiles OK.

#include <iostream>
#include <vector>

using namespace std;
typedef struct
{
    vector<int> a;
    int b;
} MYTYPE;

int main(void)
{
    MYTYPE test[]=
    {
        { {1,2,3},4},
        { {5,6},7},
        { {},8}
    };
}

If I remove the -std=c++0x flag then the compiler reports:

error: could not convert ‘{1, 2, 3}’ from ‘’ to ‘std::vector’

What is an elegant way to initialize test[] ?

unrelated: `typedef struct { ... } name;` is unecessary in C++ since `struct name { ... };` is enough to use `name` as a type (no `struct` needed like in C) — YSC, Jan 22 '18 at 15:44
Not a good duplicate. The complexity here is that the vector element is a user defined type. — Bathsheba, Jan 22 '18 at 15:48
@Holt: Feel free to submit an answer based on that "copy paste", and see how it performs in the voting. — Bathsheba, Jan 22 '18 at 16:04
@Holt: No I wouldn't downvote on that basis. I would downvote if the answer wasn't useful in my opinion, and upvote if the converse were true, which it might be. My mind is open on that point. — Bathsheba, Jan 22 '18 at 16:10

score 4 · Answer 1 · answered Jan 22 '18 at 15:39

Other than achieving some semblance of elegance at the calling site by writing a set of hideous constructors in your struct, there is no particularly elegant way of initialising an array of these structs pre-C++11.

It was for precisely these constructs that the C++11 syntax was developed.

score 2 · Accepted Answer · answered Jan 22 '18 at 15:59

With C++98, the best one can achive is probably the definition and use of an helper function:

struct Data
{
    std::vector<int> vector;
    int scalar;
};

template<std::size_t N>
Data make_data(const int (&vector)[N], int scalar)
{
    Data result;
    result.vector.assign(vector, vector+N);
    result.scalar = scalar;
    return result;
}

int main()
{
    const int vector[] = {1,2,3}; // just to annoy people using namespace std :)
    Data d = make_data(vector, 4);
    std::cout << d.vector[2] << "\n" << d.scalar << "\n";
}

live demo

But you cannot do like that in pre-c++11 `Data dd[] = { make_data({3, 2, 4}, 1) };` - I mean necessity to define extra arrays - it is something not really "elegant"?. — PiotrNycz, Jan 22 '18 at 16:32

Elegant way to initialize array of structs in C++98

2 Answers2