warning: ISO C++ forbids variable length array

问题

I have a function that takes a pointer to a floating point array. Based on other conditions, I know that pointer is actually pointing to a 2x2 OR 3x3 matrix. (in fact the memory was initially allocated as such, e.g. float M[2][2] ) The important thing is I want to make this determination in the function body, not as the function argument.

void calcMatrix( int face, float * matrixReturnAsArray )
{
    // Here, I would much rather work in natural matrix notation
    if( is2x2 )
    {
        // ### cast matrixReturnAsArray to somethingAsMatrix[2][2]
        somethingAsMatrix[0][1] = 2.002;
        // etc..
    }
    else if(is3x3)
    { //etc...
    }

}

float (somethingAsMatrix)[2] = (float ()[2]) matrixReturnAsArray;

this works fine.

here if i want mention value in subscript as a variable which will change based on some conditions.

If i do this, it showing warning: ISO C++ forbids variable length array. From this how can i dynamically create something?

回答1:

Array in C++ must have a size defined at compile-time.

If you want variable length array, use std::vector instead.

In your case, I would use template for Matrix Size and have different implementation for different matrix size (unless I misunderstood your intentions).

template<int SIZE> 
    struct Matrix {
    std::array<std::array<float, SIZE>,SIZE> m;
    std::array<float,SIZE>& operator[](int a) {
        if(a>=SIZE) {
            throw std::out_of_range("Out of range exception");
        }
        return m[a];
    }
};

template<int SIZE>
void calcMatrix(Matrix<SIZE>& matrixReturnAsArray );


template<>
void calcMatrix<2>(Matrix<2>& matrixReturnAsArray )
{
     // Code for 2x2 Matrix
    std::cout << "<2>" << std::endl;
    std::cout << matrixReturnAsArray[1][1] << std::endl;
}

template<>
void calcMatrix<3>(Matrix<3>& matrixReturnAsArray )
{
     // Code for 3x3 matrix
        std::cout << "<3>" << std::endl;
        std::cout << matrixReturnAsArray[2][2] << std::endl;
}

int main() {
   std::array<float,2> a={1,2};
   Matrix<2> m2;
   m2.m = {a,a};
   std::array<float,3> b={1,2,3};
   Matrix<3> m3;
   m3.m = {b,b,b};
   calcMatrix(m3); 
   calcMatrix(m2);
}

Since I did not defined the generic template, using any other value than 2 or 3 for size will result in an error at compile time.

Edit : Used reference to std::array instead of pointer after @Caleth's suggestion

Edit 2: Added operator [] for easy access and exception for safety

回答2:

You can pass an array by reference.

void calcMatrix(float (&matrix)[2][2])   // Only binds to 2*2 matrix
{
}
void calcMatrix(float (&matrix)[3][3])   // Only binds to 3*3 matrix
{
}

int main()
{
   float a[2][2] ={{1,2} , {3,4}};
   calcMatrix(a); 

   float b[3][3] ={{1,2,3} , {4,5,6}, {7,8,9}};
   calcMatrix(b); 
}

If you want a function that works for varying sizes you can templatize it:

template<int S>
void calcMatrix(float (&matrix)[S][S])   // Only binds to S*S matrix
{
}

回答3:

The following shows how to do the casts.

#include <stdio.h>

typedef float t2[2][2];
typedef float t3[3][3];

static void test(int is2, void * p)
{
    if(is2)
    {
        t2 *a = (t2 *)p;
        (*a)[1][1] = 1.23;
    }
    else
    {
        (*((t3*)p))[1][1] = 2.34;
    }
}

int main()
{
    printf("Hello World\n");
    t2 test2 = {{1,2},{3,4}};
    t3 test3 = {{1,2,3},{4,5,6},{7,8,9}};

    test(1,(void *)test2);
    printf("%g\n",test2[1][1]);

    test(0,(void *)test3);
    printf("%g\n",test3[1][1]);

    return 0;
}

来源：https://stackoverflow.com/questions/50722982/warning-iso-c-forbids-variable-length-array

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c++

arrays

matrix

multidimensional-array

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