C++ - Having problems defining a variable inside main() function

Question

I am trying to define a variable from an external library in C++, Visual Studio 2010. It only works when I put it outside of the main function.

This code crashes:

#include "StdAfx.h"
#include <ogdf\basic\Graph.h>
#include <ogdf\basic\graph_generators.h>

int main()
{
   ogdf::Graph g;
   ogdf::randomSimpleGraph(g, 10, 20);
   return 0;
}

It gives me an unhandheld exception: Access violation. However, if it is outside main function, it works without any problem:

#include "StdAfx.h"
#include <ogdf\basic\Graph.h>
#include <ogdf\basic\graph_generators.h>

ogdf::Graph g;

int main()
{
   ogdf::randomSimpleGraph(g, 10, 20);
   return 0;
}

Do you have any how do I fix that? I assume, that it is caused by some kind of linking problem.

EDIT: It looks like the problem is not the initialization of the variable. It throws an exception, when the application exits.

int main()
{
ogdf::Graph g; // No problem
ogdf::randomSimpleGraph(g, 10, 20); // No problem
int i; // No problem
std::cin>>i; // No problem
return 0;    // Throws an exception after read i;

}

Call stack:

The output is: First-chance exception at 0x0126788f in graphs.exe: 0xC0000005: Access violation writing location 0x00000000.

Unhandled exception at 0x0126788f in graphs.exe: 0xC0000005: Access violation writing location 0x00000000.

Answer 1

Works on my machine™.

Esoteric errors like that are often a result of binary incompability. Basically, because of different compiler/preprocessor options, effective headers that your code and the library "see" are different.

For instance, if you have a library with following header code:

class Foo
{
#ifdef FOO_DEBUG
    int debug_variable;
#endif
    int variable;
};

Library function:

void bar(Foo& foo)
{
    std::cout << foo.variable;
}

And client code:

Foo foo;
foo.variable = 666;
bar(foo);

If FOO_DEBUG is not in sync amongst client and the library, this will possibly crash and burn -- variable will have different expected offset.

In your case, I suspect one of the following may be true:

• You have built the ogdf with different compiler than your code
• If not, you ogdf and your code have different build configurations (Release vs Debug)
• Both are debug, but you have defined OGDF_DEBUG (as recommended here)
• You have different "Struct Member Alignment" setting