How to "do, if expression increased"?

Question

I have the Bitcoin source code (multithreaded) and I'm intending to add some new lines. On Bitcoin network data messages are exchanged where data are stored in vectors. I want to implement some instrunction whenever a vector increases in size:

How can I code following pattern in C++ in order to execute statements inside if:

if ("vVector.size() did not increase")
//*instruction*

?

NOTE: vVector is incremented automatically from time to time. It's actually a clone of another vector that collects things and expands without me having and willing to have control on it.

EDIT:

When I do:

if (!(vVector.size()++))
//do whatever is here

I get following error: lvalue required as increment operand

vVector is filled inside another function of the same cpp-file by a function that is declared in a separate header file.

here's a code snippet with a short description: http://hastebin.com/amisuvafab.coffee

@JonathanPotter Do you have a better idea how I can solve my problem in a short time? — Aliakbar Ahmadi, Jun 30 '15 at 12:23
You haven't said what your problem is. All you've done is post some code that can't possibly compile let alone do anything practical. — Jonathan Potter, Jun 30 '15 at 12:25
@JonathanPotter how can I put this in terms of C++ syntax: if (_"vector size increased"_) {//do something} ? — Aliakbar Ahmadi, Jun 30 '15 at 12:27
You have two answers below that answer that specific question quite well. But as they have pointed out and you seem to have ignored, vector size simply can't increase using `vector.size()++`. — Jonathan Potter, Jun 30 '15 at 12:33

score 4 · Answer 1 · edited May 23 '17 at 12:07

4

int init_size = v.size();
...
if(init_size == v.size()) {
  // do whatever is here
}

On Edit:

A std::vector's size is increased only if you insert an element to it. Thus, vVector.size()++ makes no sense in the first place.

The reason you're getting the error is because, doing if(!(vVector.size()++)) is illegal. std::vector::size member function will return an rvalue instead of an lvalue that the post-increment operator++ requires. Now on the difference between an rvalue and lvalue see this link.

edited May 23 '17 at 12:07

Community

1
1

answered Jun 30 '15 at 11:57

101010

41,839
11
94
168

it should be `v.size()` – Shreevardhan Jun 30 '15 at 11:58
@101010: Thanks. Is there an alternative to your answer? – Aliakbar Ahmadi Jun 30 '15 at 12:40
@AliakbarAhmadi if you want to check whether your vector contains any elements just `if(!Vvector.empty())` – 101010 Jun 30 '15 at 12:47
@101010 ok, I knew this one but with the hint of Shreevardhan I edited my question with a note about the code being multithreaded. – Aliakbar Ahmadi Jun 30 '15 at 12:49

Shreevardhan · Accepted Answer · 2015-06-30T12:54:12.967

3

As simple as this

int prevSize = vVector.size();
// do something here

if (vVector.size() == prevSize) {
    // do whatever is here.
}

EDIT

if (!(vVector.size()++))

This statement is plain wrong. size() method returns the value (const) of the size of vector, not the reference. So you cannot increment it. AddInventoryHonest() adds to the vector using push_back(). So the return value of size() method will change with every call to push_back. Hence this will work in this scenario.

EDIT 2: To handle the multithreaded case, use condition variable (just an example)

int prevSize = vVector.size();
// do something

std::mutex mtx;
std::unique_lock<std::mutex> lck(mtx);
std::condition_variable cv;

bool size_did_not_increase() {
    return vVector.size() == prevSize;
}

cv.wait(lck, size_did_not_increase());

edited Jun 30 '15 at 12:54

answered Jun 30 '15 at 11:57

Shreevardhan

12,233
3
36
50

Thanks! But unfortunately your answer doesn't work because as vVector.size() is adapted from time to time the if-expression will always be true, right? – Aliakbar Ahmadi Jun 30 '15 at 12:08
@AliakbarAhmadi can you show a code snippet where this doesn't work ? – Shreevardhan Jun 30 '15 at 12:11
this is a short snippet where it doesn't work: http://hastebin.com/amisuvafab.coffee – Aliakbar Ahmadi Jun 30 '15 at 12:37
@AliakbarAhmadi I assume your code isn't multithreaded, so if you don't insert anything to the vector before the if-conditional (at the comment do something), how will the size change ? – Shreevardhan Jun 30 '15 at 12:39
Actually my code is multithreaded. It's the bitcoin source code. I'll edit my question with a note to multithreading. – Aliakbar Ahmadi Jun 30 '15 at 12:42

How to "do, if expression increased"?

EDIT:

2 Answers2