C++ - Returning an array from a function

[def_77]

I'm a relatively new C++ programmer, and I've been working on a function that can return roman numeral values given an integer. The function takes an int value, and I'd like it to be able to return a char array, since you dont seem to be able to return strings. So far my function only returns the first character. The framework of it is shown below -
char rconvert(int num){
char converted = new char[MAX_SIZE];
... //Conversion taking place
... //here
return *converted;
}

int main(){
cout << rconvert(1111);
}

ive had a look to see how other people have overcome this problem but I havent found any useable solutions. Help appreciated.

[Jessehk]

One solution is to dynamically allocate memory inside the function using new, and return the pointer to the memory. At this point, you can either call delete [] on the pointer (which is prone to programmer error), or you can encapsulate the pointer into a class such as std::auto_ptr<T> or boost::shared_ptr<T> which will automatically call delete [] on the pointer once it goes out of scope.

For the boost libraries (which influence the C++ standard and are well-written), see http://www.boost.org. I believe there's an Ubuntu package in the repositories.

Here's an example:

#include <memory>

int *arraycreate( int size, int elem ) {
int *result = new int[size];

for ( int i = 0; i < size; i++ )
result[i] = elem;

return result;
}

int main() {
std::auto_ptr<int>( arraycreate( 3, 0 ) );
}


:)

[Soybean]

since you dont seem to be able to return strings.
Wait... why can't you return strings?


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

string rstring()
{
string r("this is a string");
return r;
}

int main(int argc, char *argv[])
{
string x = rstring();
cout << x << endl;
return 0;
}


Another option is to take a reference to a string (or character array) as a parameter, and use that to return your result.

[def_77]

Wait... why can't you return strings?


whoops. I tried again and realised I had forget to declare the strings namespace. Thnx for that. I now have my return working :)
Mite as well post up what I made if anyone has any suggestions. I know its a messy long winded solution, and I'm not to sure on correct programming practises yet -


#include <string.h>
using std::string;

string rconvert(long int num){
string str;
//Add M's
short int result; result = num / 1000;short int n = 0;
if(result >= 1){
for(int i = 0; i < result; i++){
str.append("M");n++;
}
num = num - (1000 * result);
}
//Add D's
result = num / 500;
if(result >= 1){
for(int i = 0; i < result; i++){
str.append("D");n++;
}
num = num - 500;
}
//Add C's
result = num / 100;
if(result >= 1){
for(int i = 0; i < result; i++){
str.append("C");n++;
}
num = num - (100 * result);
}
//Add L's
result = num / 50;
if(result >= 1){
for(int i = 0; i < result; i++){
str.append("L");n++;
}
num = num - 50;
}
//Add X's
result = num / 10;
if(result >= 1){
for(int i = 0; i < result; i++){
str.append("X");n++;
}
num = num - (10 * result);
}
//Add V's and I's
if(num == 4){ str.append("IV"); }else{
result = (num + 5) / 5;
if(result >= 1){
str.append("V");n++;
result = num - 5;
}
for(int i = 0; i < result; i++){;
str.append("I");n++;
}
}
return str;
}

[hardyn]

what about passing in a pointer, and not actully return an array?

[brooksbp]

def_77: When you dive deeping into C programming you'll learn that passing pointers to arrays instead of arrays themselves is much more efficient.

[aks44]

At this point, you can either call delete [] on the pointer (which is prone to programmer error), or you can encapsulate the pointer into a class such as std::auto_ptr<T> or boost::shared_ptr<T> which will automatically call delete [] on the pointer once it goes out of scope.

std::auto_ptr and boost::shared_ptr don't call delete[] on destruction, but delete. So they are not suitable to encapsulate an array pointer allocated by new[].

Unless you have *very* specific requirements, std::vector is the way to go when dealing with arrays.

[halfhp]

def_77: When you dive deeping into C programming you'll learn that passing pointers to arrays instead of arrays themselves is much more efficient.

Hmm...from the limited 'diving' Ive done, Ive always thought that you cant really pass an array by value unless you wrap it in a struct. So if an array is always passed by reference, what exatly is the difference? Most people will even tell you that the name of the array decays into a pointer, (which I think Ive read somewhere is also not absolutely true but still more or less equivalent)

Is this not true?

[Jessehk]

std::auto_ptr and boost::shared_ptr don't call delete[] on destruction, but delete. So they are not suitable to encapsulate an array pointer allocated by new[].

Unless you have *very* specific requirements, std::vector is the way to go when dealing with arrays.

Ah, my apologies. I had forgotten about that little problem. :(