通过引用传递结构数组

Question

I would like to pass an array of structs to a function.

struct Month {
    std::string name;
    int days;
};

Month months[12]{{"January", 31}, {"February", 28}, {"March", 31}, {"April", 30}, {"May", 31}, {"June", 30},
    {"July", 31}, {"August", 31}, {"September", 30}, {"October", 31}, {"November", 30}, {"December", 31}};

I have tried to pass the months array to functions with the parameter: Month (&months)[]

The compiler noted: "no known conversion for argument 1 from 'main()::Month [12]' to 'Month (&)[12]'".

I then attempted to pass the array by a pointer and to do this I first allocated the elements of the months array on the free-store as shown below.

Month* months[12]{new Month{"January", 31}, new Month{"February", 28}, new Month{"March", 31}, new Month{"April", 30}, new Month{"May", 31}, new Month{"June", 30},
    new Month{"July", 31}, new Month{"August", 31}, new Month{"September", 30}, new Month{"October", 31}, new Month{"November", 30}, new Month{"December", 31}};

I tried to pass this array to a function with the parameter: Month**

The compiler stated: "no known conversion for argument 1 from 'main()::Month* [12]' to 'Month**'"

I would like to know how I could pass my months array by pointer and if it is possible to pass it by reference as well.

And yes, I do know that using a vector would be a lot easier but I just started learning C++ a few days ago and I would like to become familiarized with the arrays.

The function itself is empty, I cannot write the function without knowing what my parameter will be.

void print_table(Month** months) {

}

And the function call is:

print_table(months)

After removing all extraneous code I was left with this:

struct Month;
void print_table(Month** months);

int main() {
    struct Month {
        std::string name;
        int days;
    }

    Month* months[12]{new Month{"January", 31}, new Month{"February", 28}, new Month{"March", 31}, new Month{"April", 30}, new Month{"May", 31}, new Month{"June", 30},
        new Month{"July", 31}, new Month{"August", 31}, new Month{"September", 30}, new Month{"October", 31}, new Month{"November", 30}, new Month{"December", 31}};
    print_table(months);
}

The issue was explained in the comments by john, I have also posted the corrected code.

Answer 1

Since you will treat the content of Month as constant, simply pass a rvalue reference, eg Month*&& m . Example:

void showmonths (Month*&& m, size_t nmonths)
{
    for (size_t i = 0; i < nmonths; i++)
        std::cout << std::left << std::setw(12) << m[i].name << m[i].days << '\n';
}

For further details on the declaration and use of references, see: Reference declaration

Answer 2

After reading john's comment I realized that the issue was that my struct was declared inside of the main function as shown here:

struct Month;
void print_table(Month** months);

int main() {
    struct Month {
        std::string name;
        int days;
    }

    Month* months[12]{new Month{"January", 31}, new Month{"February", 28}, new Month{"March", 31}, new Month{"April", 30}, new Month{"May", 31}, new Month{"June", 30},
        new Month{"July", 31}, new Month{"August", 31}, new Month{"September", 30}, new Month{"October", 31}, new Month{"November", 30}, new Month{"December", 31}};
    print_table(months);
}

I edited to declare the struct outside of the main function and it worked. Thanks!

struct Month {
    std::string name;
    int days;
}
void print_table(Month** months);

int main() {
    Month* months[12]{new Month{"January", 31}, new Month{"February", 28}, new Month{"March", 31}, new Month{"April", 30}, new Month{"May", 31}, new Month{"June", 30},
        new Month{"July", 31}, new Month{"August", 31}, new Month{"September", 30}, new Month{"October", 31}, new Month{"November", 30}, new Month{"December", 31}};
    print_table(months);
}

Answer 3

Try this

Month months[10];
//like this you can pass
print_table(months);

Pass to this function

void print_table(struct Month * months)
{
   //code here
}