C++ Tip #1: constant references

[vladislavbelov]

We talked with @elexis about constant references and I gave an example why passing by value may be better than passing by constant reference. During todays refactoring I met another important things that you need to know about constant references.

Aliasing

Take a look at the following code. Do you see a problem? (It's our code from ps/Shapes.h).

// Interface
class CSize
{
public:
    // ...
    void operator/=(const float& a);
  
    // ...
public:
    float cx, cy;
}

// Implementation
void CSize::operator/=(const float& a)
{
    cx /= a;
    cy /= a;
}

If not, would the following usage example help you?

CSize size(2560, 1440);
// Normalize the size.
if (size.cx)
    size /= size.cx;
debug_printf("%f %f", size.cx, size.cy);

You'll get:

1.0 1440.0

Because the a references to cx, and the first line of the operator modifies the cx. And in the next we just divide the cy by 1. It may happen in each case where we get the same class.

I fixed the problem in D1809.

Lifetime

Another important thing is a lifetime. Let's take another look at another example (fictional, because I didn't find more detailed example in our code yet):

std::vector<Node> nodes;
// ...
duplicate(nodes[0], 10);
// ...
void duplicate(const Node& node, size_t times)
{
    for (size_t i = 0; i < times; ++i)
        nodes.push_back(node);
}

From first look it seems ok. But, if you know how std::vector works then you know, that on each push_back std::vector can reallocate array to extend it. And then all iterators and raw pointers are invalid, including our constant reference. So after few duplication calls it may contain a trash.

So, you need to be careful with such cases.

[GunChleoc]

For the duplicate function, resize and fill from the standard library should do the trick too - of course, with a copy instead of a reference.

http://www.cplusplus.com/reference/vector/vector/resize/

http://www.cplusplus.com/reference/algorithm/fill/

[vladislavbelov]

7 hours ago, GunChleoc said:

For the duplicate function, resize and fill from the standard library should do the trick too - of course, with a copy instead of a reference.

http://www.cplusplus.com/reference/vector/vector/resize/

http://www.cplusplus.com/reference/algorithm/fill/

Yeah, every function that may invalidate the constant reference is potentially dangerous.