Question

I’ve got a quasi-singleton class (quasi-singleton in that most use refers to a single object that is a function static, but users can also construct their own local copy for short-term use) that I would like to have write to cout from its destructor, and wanted to know if cout was guaranteed to be available during the static deinitialization phase following program termination. From this question it seems that the answer is yes (function static initialized objects should have their destructors called in reverse order from when they were constructed, which should be after cout was set up), but I wanted to check.

// Count calls to a logging function from some point in our code, to determine
// how many times it gets executed during a run, then report calls at the end
// of the program.  A quick-and-dirty way of determining how often we execute
// code.
class callCounter;
class callCounter {
public:
    callCounter() : has_calls_(false), counts_() {}
    ~callCounter () {report(std::cout);}
    void logCall(const std::string callSite);
    void report(std::ostream &os);
    void reset();
    static callCounter *theCounter();
private:
    typedef std::map<std::string, callCount> COUNTMAP;
    bool has_calls_;
    COUNTMAP counts_;
};

callCounter *callCounter::theCounter()
{
    static callCounter theCounts;
    return &theCounts;
}

Typical usage would be:

callCounter::theCounter()->logCall("foo");

So is this usage of cout in the destructor guaranteed safe (at least as far as using cout itself is concerned – arguably to be perfectly safe, I should wrap the report in a try block in case the write throws an exception.)?

Answer 1

Short Answer: Yes

Long Answer: Yes

There are a couple of tricks that the standard library uses to make sure that std::cout (and std::cin/std:cerr) are available before any other objects (But you must #include <iostream> in the translation unit). Because they are created first (the reverse order of destruction rule guarantees) they are destroyed after your objects.

See: n3242 (Practically the C++0x standard Pub: 2011-02-28)

27.4 Standard iostream objects

Paragraph 2

The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution. The objects are not destroyed during program execution. The results of including in a translation unit shall be as if <iostream> defined an instance of ios_base::Init with static storage duration. Similarly, the entire program shall behave as if there were at least one instance of ios_base::Init with static storage duration.

Added highlighting.

From n1804 (which is basically the 2003 standard: Pub: 2005-04-27)

27.3 Standard iostream objects

Paragraph 2

Mixing operations on corresponding wide- and narrow-character streams follows the same semantics as mixing such operations on FILEs, as specified in Amendment 1 of the ISO C standard. The objects are constructed, and the associations are established at some time prior to or during first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution. The objects are not destroyed during program execution²⁸⁰.

Unfortunately the bit that would make it a guarantee is in the footnotes (which are non-normative (ie its not guaranteed just what the implementer should try and achieve).

With the footnotes:

280) Constructors and destructors for static objects can access these objects to read input from stdin or write output to stdout or stderr.