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std::ios_base:: register_callback

From cppreference.net
C++
Input/output library
std::ios_base
void register_callback ( event_callback function, int index ) ;

Registers a user-defined function which will be called by imbue() , std::basic_ios::copyfmt() and ~ios_base() . Every registered callback is called every time: the event type (a value of type event ) is passed as its first argument, and may be used to distinguish between the callers.

The callbacks are called in the reverse order of registration (in other words, register_callback() pushes a callback pair on the callback stack). If register_callback() is called from within a callback function to add a new callback, the new callback is only called on the next event.

The user-defined callback function is not allowed to throw exceptions.

Contents

Parameters

function - the function which will be called on event, supplied as a function pointer of type event_callback
index - custom parameter which will be passed to the function

Return value

(none)

Notes

Once registered, a callback cannot be deregistered: it remains a part of the stream object for the rest of its lifetime. If the behavior of a callback needs to change, it may be controlled through iword() or pword() .

If the same function is registered multiple times, it is called multiple times.

The integer value that is stored together with the callback is typically an index obtained from xalloc() .

Example

Demonstrates the use of register_callback to update locale-dependent cached values that are used by a custom output operator.

Run this code 
#include <functional>
#include <iostream>
#include <locale>
// Cached locale-specific message and its hash
typedef std::pair<std::string, std::size_t> cache_t;
// Populate the cached message and its hash from the locale
void update_cache(cache_t& cache, std::locale loc)
{
    auto& fct = std::use_facet< std::messages<char> >(loc);
    std::messages_base::catalog cat = fct.open("sed", loc);
    cache.first = cat < 0 ? "" : fct.get(cat, 0, 0, "Memory exhausted");
    cache.second = std::hash<std::string>()(cache.first);
}
// Update the cache if the locale changed
void true_callback(std::ios_base::event evt, std::ios_base& str, int idx)
{
    if (evt == std::ios_base::imbue_event) 
    {
        cache_t* ptr = static_cast<cache_t*>(str.pword(idx));
        update_cache(*ptr, str.getloc());
    }
}
// Registers the cache in pword() and sets up the callback
struct CacheSetup
{
    CacheSetup(std::ostream& os, std::ios_base::event_callback f, cache_t* cache)
    {
        int index = std::ostream::xalloc();
        os.pword(index) = cache; // Store pointer to cache in the stream
        os.register_callback(f, index); // Store callback and the index to the pointer
        update_cache(*cache, os.getloc()); // Initialize cache
    };
};
// Some custom class 
struct S {};
// Some custom class's operator<< that needs fast access to hashed message
std::ostream& operator<<(std::ostream& os, const S&)
{
    static cache_t cache;
    static CacheSetup setup(os, true_callback, &cache);
    return os << cache.first << " : " << cache.second;
}
int main()
{
    std::locale loc("en_US.utf8");
    S s;
    std::cout.imbue(loc);
    std::cout << s << '\n';
    std::cout.imbue(std::locale(loc, new std::messages_byname<char>("de_DE.utf8")));
    std::cout << s << '\n';
    std::cout.imbue(std::locale(loc, new std::messages_byname<char>("ja_JP.utf8")));
    std::cout << s << '\n';
    std::cout.imbue(std::locale(loc, new std::messages_byname<char>("ru_RU.utf8")));
    std::cout << s << '\n';
}

Output: 

Memory exhausted : 2,295,079,096
Speicher erschöpft : 3,139,423,551
メモリーが足りません : 3,837,351,114
Память исчерпана : 3,742,732,851