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Spencer Allen McMillan

Memory Allocation

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Hello, I'm learning memory allocation online right now and I'm having some issues. I'm trying to create the size of an array in run-time and I seem to be having some issues. When I code it all inline it works fine, but when I put it in a function I seem to be doing something wrong. I keep getting this error on Visual Studio: [b]error C2082: redefinition of formal parameter 'a'[/b]

Here's the code.

[CODE]
#include<iostream>
#include<cstdlib>
#include<ctime>
using namespace std;
void randomArrayFill( int *a, int size )
{
int *a = new int[size];
for( int i = 0; i < size; i++ )
{
a[i] = rand() % 100;
}
}

int main()
{
//----Init--------------------------------------------------------------
int size = 0;
int *a = 0;
srand(0);
//----End Init----------------------------------------------------------
cout << "Enter the size of an array to create: ";
cin >> size;

randomArrayFill( a, size );
for( int i = 0; i < size; i++ )
{
cout << a[i];
}
cin.get();
cin.get();
return 0;
}
[/CODE]

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I'm stumped. It compiles fine, but when it gets to the for function in main that prints off the array it says I don't have access to it. Here's the revised code. [CODE]
#include<iostream>
#include<cstdlib>
#include<ctime>
using namespace std;
void randomArrayFill( int *a, int size )
{
a = new int[size];
for( int i = 0; i < size; i++ )
{
a[i] = rand() % 100;
}
}

int main()
{
//----Init--------------------------------------------------------------
int size = 0;
int *a = 0;
srand(0);
//----End Init----------------------------------------------------------

cout << "Enter the size of an array to create: ";
cin >> size;

randomArrayFill( a, size );

for( int i = 0; i < size; i++ )
{
cout << a[i] << " ";
}
cin.get();
cin.get();
return 0;
}
[/CODE]

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Brother Bob    10344
The variable [i]a[/i] in [i]main[/i] is different from the variable [i]a[/i] in [i]randomArrayFill[/i] because you're passing it by value. You need to pass a reference or a pointer to the variable you want to modify.
[source]
void randomArrayFill(int *&a, int size)
{
...
}
[/source]
If you're passing the pointer by value, you're only modifying a copy of the pointer which is local to the function. With a reference to the pointer, you can modify the original pointer in [i]main[/i].

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osmanb    2082
It's worth pointing out that the issue you're having here is not unique to pointers. You need to understand pass-by-value and pass-by-reference, even without pointers getting involved. What does this code print?

[source lang="cpp"]
void changeValue( int a )
{
a = 42;
}

int main()
{
int a = 7;
changeValue( a );
cout &lt;&lt; a;
return 0;
}
[/source] Edited by osmanb

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rip-off    10979
Note that it is idiomatic to use the return value of a function where possible:
[code]
int *randomArrayFill( int size )
{
int *result = new int[size];
for( int i = 0; i < size; i++ )
{
a[i] = rand() % 100;
}
return result;
}
[/code]

Once you have learned about memory allocation, you can then go on to use the standard library containers to avoid unnecessary manual memory management:
[code]
#include <vector>

std::vector<int> randomArrayFill( int size )
{
std::vector<int> result;
for( int i = 0; i < size; i++ )
{
a.push_back(rand() % 100);
}
return result;
}
[/code]
This will prevent the memory leaks NightCreature83 mentioned.

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BeerNutts    4401
Be aware, rip-off has a typo in his first example. The line:
a[i] = rand() % 100;
should be
result[i] = rand() % 100;

Like this
[code]
int *randomArrayFill( int size )
{
int *result = new int[size];
for( int i = 0; i < size; i++ )
{
result[i] = rand() % 100;
}
return result;
}
[/code]

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NightCreature83    5006
[quote name='rip-off' timestamp='1341223020' post='4954844']
Note that it is idiomatic to use the return value of a function where possible:
...
Once you have learned about memory allocation, you can then go on to use the standard library containers to avoid unnecessary manual memory management:
...
This will prevent the memory leaks NightCreature83 mentioned.
[/quote]
Just as a side note you have to learn memory management and all it's little caveats before you move onto the standard containers as not all shops use the standard containers. Some shops avoid the standard library if they can for various reasons and thus you are off having to know memory managment, also it gives you an insight into how the Standard Containers work in the background.

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mhagain    13430
One note about memory management is that - depending on the APIs available to you - manual memory management may very well turn out to be more suitable for you. E.g. on Windows, a single HeapDestroy call is all that is needed to free a whole chunk of separate allocations, and without needing to keep track of individual allocations within that chunk.

Of course when learning you should treat this as an "advanced topic" and keep away from it - learn the proper way first. But do be aware that it does exist and can occasionally be a preferable choice - just don't do it yet.

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