wxEVT_BUILD_STARTING event

[sdolim]

Eran is correct. The temporary value is destroyed at the end of the statement:

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event.SetClientData(&(proj->GetName()));

But this is before the statement that actually calls ProcessEvent() to send the event to the plugins. The temporary value has been freed by that point.

Also, using the address of a local variable is OK in this case since the event handling is synchronous: ProcessEvent() does not return till all receivers have processed the event.

[asterisc]

Why is this working?

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#include <iostream>
#include <string>

using namespace std;

string msg()
{
	string v = "10";
	return v;
}

void print( string *msg )
{
	cout << msg->c_str() << endl;
}

int main(int argc, char* argv[])
{
	print( &msg() );

	return 0;
}

There are exceptions described in C++ standard.

Maybe I didn't get the problem right, but it's sure that the temporary isn't "undefined" freed

[sdolim]

In this case, the temporary that holds the return value of msg() stays in scope until the end of the statement, when print() has returned.

This is different from the event-sending case, where there are two separate statements:

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event.SetClientData(...);
ProcessEvent(event);

In that case, a temporary used in the first function call has gone out of scope before ProcessEvent() is called. But using a local variable instead ensures that the value stays in scope through the end of the function (or block) containing both statements.

[eranif]

Here is a complete sample that show the problem:

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#include <iostream>
#include <string>

using namespace std;

struct event {
	void *d;
};

class cls {
public:	
	string s;
	
public:
	cls(){
		cout << "ctor" << endl;
	}
	
	~cls(){
		cout << "dtor" << endl;
	}
};

cls msg()
{
	cls c;
	c.s = "10";
	return c;
}

void print( event *e )
{
	cout << ((cls*)e->d)->s.c_str();
}

int main(int argc, char* argv[])
{
	event e;
	e.d = (void*)&msg();
	print( &e );
	return 0;
}

Running this code gives this output:

ctor
dtor
10

which means that print() was invoked with 'e' pointing to an invalid pointer.
The reason that it does not crash is just because that the memory was not reused again - pure luck

Eran