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Conversion from boost::shared_ptr to std::shared_ptr?

Based on janm’s response at first I did this: template<class T> std::shared_ptr<T> to_std(const boost::shared_ptr<T> &p) { return std::shared_ptr<T>(p.get(), [p](…) mutable { p.reset(); }); } template<class T> boost::shared_ptr<T> to_boost(const std::shared_ptr<T> &p) { return boost::shared_ptr<T>(p.get(), [p](…) mutable { p.reset(); }); } But then I realized I could do this instead: namespace { template<class SharedPointer> struct Holder { … Read more

map with incomplete value type

The standard requires that, all types used in template components of the standard library must be complete, unless otherwise stated. Thus libc++’s error is correct. Using an incomplete type is undefined behavior (§17.6.4.8[res.on.functions]/2), so libstdc++ accepting it isn’t wrong either. You could use a pointer object to construct the complete type as you did. But … Read more

Forward Declaration of variables/classes in std namespace

You can forward declare your own classes in header files to save compilation time. But you can’t for classes in namespace std. According to the C++11 standard, 17.6.4.2.1: The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. … Read more

How to make sure that std::random_shuffle always produces a different result?

std::random_shuffle has two forms. One that takes 2 arguments (begin/end iterators), and one that takes 3 (begin/end iterator and a random generator). The first form uses std::rand(), so you would use std::srand() to seed it’s random number generator. You can also use the 3-argument version and provide the RNG yourself.

What data structure is inside std::map in C++?

Step debug into g++ 6.4 stdlibc++ source Did you know that on Ubuntu’s 16.04 default g++-6 package or a GCC 6.4 build from source you can step into the C++ library without any further setup? By doing that we easily conclude that a Red-black tree used in this implementation. This makes sense, since std::map, unlike … Read more

I want to kill a std::thread using its thread object? [duplicate]

@bamboon’s answer is good, however I feel this deserves a stronger statement. Whatever the language you use, your program will acquire and release resources: memory, file descriptors, … For simple programs that are fired in one shots, leaking resources does not matter much: when the program ends modern OSes automatically take the resources back; however … Read more

std::ofstream, check if file exists before writing

This is one of my favorite tuck-away functions I keep on hand for multiple uses. #include <sys/stat.h> // Function: fileExists /** * Check if a file exists * * @param[in] filename – the name of the file to check * * @return true if the file exists, else false */ bool fileExists(const std::string& filename) { … Read more

What’s the time complexity of iterating through a std::set/std::map?

In the draft C++11 standard N3337 the answer can be found in § 24.2.1 paragraph 8: All the categories of iterators require only those functions that are realizable for a given category in constant time (amortized). Since each operation on an iterator must be constant time, iterating through n elements must be O(n).

what does `using std::swap` inside the body of a class method implementation mean?

This mechanism is normally used in templated code, i.e. template <typename Value> class Foo. Now the question is which swap to use. std::swap<Value> will work, but it might not be ideal. There’s a good chance that there’s a better overload of swap for type Value, but in which namespace would that be? It’s almost certainly … Read more

C++ std::tuple order of destruction

I’ll offer a life lesson I’ve learned, rather than a direct answer, in response to your question: If you can formulate, for multiple alternatives, a reasonable argument for why that alternative should be the one mandated by the standard – then you should not assume any of them is mandated (even if one of them … Read more