Thread: Entering size of an array

The line "class *names;" is a declaration of a variable of named "names" of type "pointer to class".

However, "class" is a keyword in C++, and cannot be used as a type name. Hence the error message.

Read up on how you specify classes, before trying to create arrays.

The example of "Dynamic Arrays" in that link you gave is about creating an array of MyClass. That whole page assumes you understand how to specify a new type. That is because, if you want to create an array of X, it is necessary (in C++ at least) that X be specified first.

names is suppose to be a string array right?

Code:

std::string * names;
names = new std::string[order];

From what I understood you are trying to make a program which asks you for the size of an array which will contain strings. In that case try with this:

Code:

#include <iostream>

using namespace std;

int main()
{
    int o; //o stands for order
    int i;
    char names[o]; //the array "names" is the same size as "o"
    
    cout <<"Insert the order: "<<endl;
    cin >> o;
    
    for(i=0; i<o; i++)
    {
        cout <<"Insert the<<i<<element: "<<endl;
        cin >> names[i];
    }
    
    system("pause");
    return 0;
}

Originally Posted by godly 20

From what I understood you are trying to make a program which asks you for the size of an array which will contain strings. In that case try with this:

Code:

#include <iostream>

using namespace std;

int main()
{
    int o; //o stands for order
    int i;
    char names[o]; //the array "names" is the same size as "o"
    
    cout <<"Insert the order: "<<endl;
    cin >> o;
    
    for(i=0; i<o; i++)
    {
        cout <<"Insert the<<i<<element: "<<endl;
        cin >> names[i];
    }
    
    system("pause");
    return 0;
}

'o' wasn't initiaized yet, how could you use it to declare the size for names???? This sets up a world of hurt for debugging.

Look up <vector>, use it as your container class, the size of vector and be expand or contracted by using the member function.

and even if it was initialized you can't declare an array like that without a constant. to declare arrays with non constant variables use the new keyword

Originally Posted by codeprada

and even if it was initialized you can't declare an array like that without a constant. to declare arrays with non constant variables use the new keyword

You could if your compiler support variable size array.

Originally Posted by nimitzhunter

You could if your compiler support variable size array.

none of my compilers ever did

Originally Posted by CatastropheNoob

I have also tried setting up new variables each time in a loop, but I cannot find a way to find a new name for the variables each time the loop goes round.

Why would a user of the program care what a variable is named? That name doesn't exist when you compile the program, it's just a memory address.

What do you actually want your program to do once you allocate an array of objects?

I'd recommend using an std::vector first, then learn about pointers & memory allocation once you have a better understanding of how to program in C++. An std::vector is like an array that grows and shrinks automatically (it handles all the memory allocation internally).

Originally Posted by nimitzhunter

You could if your compiler support variable size array.

g++ 4.5.2 supports it here, but... VLAs are not a feature of C++. If it works, then only because you compile in non-pedantic mode with compiler specific extensions enabled - and it is indeed an extension of GCC.

Again, VLAs are a feature of C99, not C++.
In C++ you should use some container like vector, list, map or whatever suits the needs. If you really want, you still can manage your memory by hand, using the "new" keyword as was already said by codeprada. (remember to always delete what you newed

I have always used pointers to accomplish this.... I usually deal with 2d arrays though it would be the same as a vector of vectors of class instantiations. Which I would think would be easier to change size whenever you want with my method I would *probably* end up increasing size by copying then adding so vectors would be the way you want to go if it will be completely dynamic size changinging from here to there. I would think if you just wanted to ask something like "how many inputs?" User enters some number and you work off that number which will not change during this run of the program pointers would be more efficent memory wise/cpu cycle wise as you don't have the overhead of the vector class all be it useful when you want to change things on the fly.

The vector overhead is negligible in 99% of the case, though. Don't avoid it unless you have measured a significant performance hit in it.

Overhead is overhead I have programed for IC chips with only a few kb worth of storage mostly written in asm mind you, but pointers work just as well in a class as a vector would and with a proper destructor you don't run into memory leaks unless your program already has them.