STL vector : can not use begin() and end() on function return

[linq936]

Hi,
I have some code like this,

vector<MyNode*getNodes() {
return theNodes;
}

int func() {
vector<MyNode*nodes;
nodes.insert(nodes.end(), getNodes().begin(), getNodes().end());

}
This code crashes on Windows using VC7 compiler. In debuging it I
find that I have to change the code to:

int func() {
vector<MyNode*nodes, tmp_nodes;
tmp_nodes=getNodes();
nodes.insert(nodes.end(), tmp_nodes.begin(), tmp_nodes.end());
}
It looks like I can use begin() and end() directly to a function
return, but I am wondering why?

[David Harmon]

On 10 Sep 2006 20:55:36 -0700 in comp.lang.c++, li*****@hotmail.com
wrote,

vector<MyNode*getNodes() {
return theNodes;
}

The above returns a copy of "theNodes" to the caller.

int func() {
vector<MyNode*nodes;
nodes.insert(nodes.end(), getNodes().begin(), getNodes().end());

}

The above calls getNodes twice, getting two copies of theNodes. It
then tries to compare the .begin() of one of them to the .end() of
the other one. They are not comparable since they belong to two
different vectors. Boom.

[tragomaskhalos]

li*****@hotmail.com wrote:

Hi,
I have some code like this,

vector<MyNode*getNodes() {
return theNodes;
}

int func() {
vector<MyNode*nodes;
nodes.insert(nodes.end(), getNodes().begin(), getNodes().end());

}

I suspect you want this:

vector<MyNode*const & getNodes() { // nb return type
return theNodes;
}

void func() {
vector<MyNode*nodes;
nodes.insert(nodes.end(), getNodes().begin(), getNodes().end());
}

[Clark S. Cox III]

li*****@hotmail.com wrote:

Hi,
I have some code like this,

vector<MyNode*getNodes() {
return theNodes;
}

int func() {
vector<MyNode*nodes;
nodes.insert(nodes.end(), getNodes().begin(), getNodes().end());

}
This code crashes on Windows using VC7 compiler. In debuging it I
find that I have to change the code to:

int func() {
vector<MyNode*nodes, tmp_nodes;
tmp_nodes=getNodes();
nodes.insert(nodes.end(), tmp_nodes.begin(), tmp_nodes.end());
}
It looks like I can use begin() and end() directly to a function
return, but I am wondering why?

Each time you call getNodes() in that last line of func(), you get a new
copy of theNodes.

You basically have two choices:

1) Use a temporary variable as you do above.
2) Change getNodes to return a reference, like so:
const vector<MyNode*&getNodes() {
return theNodes;
}

#2 has the advantage that it doesn't cause a new copy of theNodes to be
made.

--
Clark S. Cox III
cl*******@gmail.com