如何正确定义和使用返回类型为用户定义结构的函数？ （在C中）

Question

I did my best to search for help on this but given the keywords for this problem, I kept getting basic tutorials that didn't have my specific problem.

I was given a C library to use that defined "Matrix" as a structure:

typedef struct 
{
int height, width;
double **map;
} Matrix;

I am writing a function that needs to output a matrix. My code currently looks like:

Matrix convolve(Matrix data, Matrix filter) //this is line 28 btw
{
    Matrix out;
    <code>
    return out
}

So far everything compiles. Then I try to use it in my main() block:

int main()
{
    double a1[4][5] = <a ton of numbers>
    double f[3][3] = <a ton of other numbers>

    Matrix m1 = createMatrixFromArray(&a1[0][0],4,5); //from given C library
    Matrix mf = createMatrixFromArray(&f[0][0],3,3);

    //This is the line that doesn't compile, which is line 14:
    Matrix m2 = convolve(m1, mf);
}

Then Developer Command Prompt for VS 2017 says:

(14) error C2440: 'initializing': cannot convert from 'int' to Matrix'
(28) error C2371: 'convolve': redefinition; different basic types

What am I doing wrong? Thank you in advance for your help.

Answer 1

It's difficult to be sure because you didn't post your complete code , but the combination of these two error messages strongly suggests that the compiler saw a use of the convolve function before it was declared. For historical reasons, when a compiler sees a function that it doesn't know about, it assumes that this function returns int , rather than complaining and aborting the compilation. From the code you show, that first use would be line 14. Since the returned int is assigned to a Matrix , the compiler tried to convert the value, but there's no conversion between int and Matrix .

Later, on line 28, the compiler saw the definition of the convolve function, this time returning Matrix , which is incompatible with the previous (implicit) declaration.

The solution, in your case, is to define the function before it is used. In C, and more generally in most programming languages, you need to define (or at least declare) things before they are used. The main function should always be the last one in your source file since it uses other functions but no other function uses it¹.

If the function was defined in a different source file, you would need to declare it in a header file ( .h ), and include the header file in the .c files where the function is used.

Any halfway decent compiler would at least explicitly warn about implicit declarations: they're permitted, but they're a bad idea. Visual Studio can do it, but I think you need to raise the warning level from the default.

¹ _{Except in highly unusual programs that call main recursively.}