Re: Order of evaluation of expression

Ulrich Eckhardt <>
Tue, 03 Jun 2008 18:25:08 +0200
Paul wrote:

I am thinking of implementing a manipulator for a stream whose role would
be to lock the stream, run some prefix code (output a few initial values)
and then run some suffix code (flush and unlock the stream).


struct Lock {
    void lock();
    void unlock();

struct Latch {
    Latch(Lock& lck) : lock(lck) { lock.lock(); }
    ~Latch() { lock.unlock(); }
    Lock& lock;

inline std::wostream& operator <<(std::wostream& os, const Latch&)
    return os;

int main()
    Lock lck;

    std::wcout << Latch(lck) << L"Something else 1.\n"
               << L"Something else 2.\n";

    return 0;

The code seems to be working all right. Is there nothing wrong with this

I can't see anything why this shouldn't work.

My way of thinking is this. The arguments for the call to the << operator
will be created before the call. The order of creation is unspecified but
this does not matter - it is the output itself that does.

The Latch object will be created before any call to operator<< takes place
(because it is an argument), otherwise its creation time is unspecified. It
will be destroyed after the last call to operator<< because only there is a
sequence point where temporaries are destroyed.

In other words, your design will work correctly. However, there is one thing
there which I don't like, and that is the fact that it slightly obfuscates
what is going on. Consider these alternatives:

1. Use the comma operator:

  Latch(lck), std::wcout << L"Fou" << L"Barre!" << std::endl;

This has the same guarantees, only that the Latch will be created before the
right side of the comma is evaluated.

2. Use a wrapper around the stream:

  MyLatch(std::wcout) << L"Fou" << L"Barre!" << std::endl;

The MyLatch would have a templated, overloaded operator<< which then returns
just the stream.

Note that if you want a prefix at the beginning of a line, you might need
something like this:

  struct wrapper: std::wostringstream {
    wrapper(std::wostream& target): m_target(target) {}
    ~wrapper() {
      std::wstring const& s = this->str();
      // chop into lines and forward to m_target
    std::wostream& m_target;

This can also keep the locking time of the target stream a bit lower,
because you only feed it complete lines at a time instead of formatting the
output while the stream is already locked. You will have to copy the format
flags though, if that is important.


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