在二维 C 数组中转换 std::vector 数组

Question

On a project I'm working on, I need some dynamic allocation due to the size of the used data not been known in advance. std::vector seems perfect for this use case. However, due to the software environnement, I can not use "modern" C++ in the headers. I would like to convert this vectors array to be used in fuction with compliant headers.

Quick example:

    void func(int tab[][]/*Vector can not be used here*/){/*Do things*/}
    
    int main(){
        std::vector<int> vecTab[6/*Fixed, prior known, dimension*/];
        
        //Adding a random number of values in each vector (equal number in each one)
        //Transformation of vecTab
    
        func(vecTabMod);

        return 1;
    }

There is a lot of similar questions on this site, none of them really adressing bi-dimensionnal arrays.

Bonus point: no reallocation, access through pointers

Answer 1

You'll need to copy the data pointers into a separate array so that the type and layout matches what the funciton expects. This can be done without heap allocation since the size of this array is fixed.

int* vecTabMod[6];
std::transform(std::begin(vecTab), std::end(vecTab), std::begin(vecTabMod),
               [](auto& v) { return v.data(); });
func(vecTabMod);

Answer 2

std::vector is worst choice for this soultion. Using dynamic arrays is better.

Anyway you can use this code:

#include <vector>
#include <iostream>

int func(uint32_t firstDimensionSize, uint32_t* secoundDimensionSizes, int** tab){
    int sum = 0;
    for(uint32_t i = 0; i < firstDimensionSize; i++){
        for(uint32_t j = 0; j < secoundDimensionSizes[i]; j++){
            sum += tab[i][j];
        }
    }
    return sum;
}

int main(){
    std::vector<int> vecTab[6];
    vecTab[0].push_back(2);
    vecTab[0].push_back(5);
    vecTab[3].push_back(43);

    // Calculate count of elements in non dynamically arrays
    uint32_t firstDimensionSize = (sizeof(vecTab) / sizeof((vecTab)[0]));
    uint32_t* secoundDimensionSizes = new uint32_t[firstDimensionSize];
    int**tab = new int*[firstDimensionSize];
    for(uint32_t i = 0; i < firstDimensionSize; i++){
        secoundDimensionSizes[i] = vecTab[i].size();
        tab[i] = &(vecTab[i][0]);
    }

    std::cout << func(firstDimensionSize, secoundDimensionSizes, tab) << std::endl;
    delete[] secoundDimensionSizes;
    delete[] tab;
    system("pause");

}