Passing Arrays as parameters

[Falconus]

I am trying to create a function that returns the length of an array. I am running into endless problems, I believe because it will only point at the array, not actually refer to the entire array. Here is my code, which consistently returns "1". Does anybody know how I can do this? Thanks!

int arraylength(string input_array[]){
string* array = input_array;
return (sizeof(input_array) / sizeof(input_array[0]));
}

[Zhuge]

When you pass an array to a function, it decays into a pointer to the first element. So sizeof(input_array) returns the size of a pointer on your system, not the size of the entire array.

Addition:
I believe this code can do what you want.

template<class T, size_t size> size_t array_length(T(&)[size]) {
    return size;
}

Please ask if you have any more questions.

[Falconus]

Okay, thank you. I've never used a template before, how do I call it? Can I put it in a header file? Thanks.

[Zhuge]

You should be able to just use it like a normal function; the compiler should be able to figure out the template parameters on it's own. Just make sure you pass it an actual array, not a pointer.

[Falconus]

I think I am almost there, but I am getting an error:

g++ test.c++ functions.h; ./a.out
test.c++: In function ‘int main()’:
test.c++:13: error: no matching function for call to ‘array_length(std::string*&)’

Would that mean that I am not sending it an actual array?

[richardforc]

Say you have string a[10];
Just use array_length(a);

Also check with your function declaration whether you created same as how Zhuge mentioned.

Thanks.

[Falconus]

Okay, thank you. If I want to define the array in a separate function (say in a header file) and be able to get the length in the main program, is there a way to do that, or would I be better to append a special character in the last element or something along those lines?

[Zhuge]

You'll probably need to. Anytime you pass an array, it will decay to a pointer and the compiler will no longer know how long it is. Depending on what you are doing, however, it could be much easier to simply use a vector instead.

[krishnendu]

Always use std::vector<> object to create arrays. The following code creates an array of double.

#include <iostream>
#include <vector>
using namespace std;

int main (void)
{
        const double numbers[] = {0, 1, 2, 3, 4};
        vector<double> theArray (numbers, numbers + 5);

        /**
        Now you can pass theArray to anywhere.
        Its size information will go with it.
        You can do many more with it.
        Refer to vector section in STL documentation.
        **/

        cout << "The length of the array is " << theArray.size () << endl;

        return 0;
}

Change 'vector<double>' to 'vector<char>' if you want to create an array of character. However you will not certainly do this. Because C++ STL provides std::string as a pre-fabricated class which inherently contains an array of character. Thus the following code will print the length of a string.

#include <string>
#include <iostream>
using namespace std;

int main (void)
{
        string str = "What is the length of this string?";
        cout << "Length = " << str.size () << endl;
}

If you want to pass the string to a function as an argument, pass its reference.

int findStringLength (const std::string &str)
{
        return str.size ();
}

or

int findStringLength (const std::string *str)
{
        return str->size ();
}

Note that, the reference is sent as 'const std::string &' or 'const std::string *'. The object is not copied. The 'const' is recommended because the value of the string is not intended to be changed inside the function.

Hope the information is enough. Enjoy coding :)

[Falconus]

Okay, thank you. It's been awhile since I've done C++, so I had completely forgotten about the vectors :-/.