c++-standard-library

问题 I wonder where on my file system I find the headers of the C++ Standard library. In particular I am looking for the definition of the vector template. I searched in /usr/include/ and various subdirectories. I also tried 'locate vector.h' which brought up many implementations of vectors, but not the standard one. What am I missing? (The distribution is Gentoo) Background: I'm profiling a library that iterates over vector's most of the time and gprof shows that most of the time is spent in std:

问题 In N3421 - Making Operator Functors greater<>, the new specialization for the std function objects is: template <> struct plus<void> { template <class T, class U> auto operator()(T&& t, U&& u) const -> decltype(std::forward<T>(t) + std::forward<U>(u)); }; instead of template <> struct plus<void> { template <class T, class U> auto operator()(T&& t, U&& u) const noexcept(noexcept(decltype(std::forward<T>(t) + std::forward<U>(u)) (std::move(std::forward<T>(t) + std::forward<U>(u))))) -> decltype

问题 A large amount of functionality is duplicated between standard c++ and Qt. At some point it seems logical but many times it looks foolish. Like I feel like doing a new programming language, learning things which I already know. e.g. using QFile. Also if I do it all Qt way and suppose now I want to move out of Qt framework it will be too much tedious to rewrite that code. OTOH I like Qt because it provides me with libraries which otherwise I would have to fish myself like webkit, database

问题 A large amount of functionality is duplicated between standard c++ and Qt. At some point it seems logical but many times it looks foolish. Like I feel like doing a new programming language, learning things which I already know. e.g. using QFile. Also if I do it all Qt way and suppose now I want to move out of Qt framework it will be too much tedious to rewrite that code. OTOH I like Qt because it provides me with libraries which otherwise I would have to fish myself like webkit, database

问题 I like vectors a lot. They're nifty and fast. But I know this thing called a valarray exists. Why would I use a valarray instead of a vector? I know valarrays have some syntactic sugar, but other than that, when are they useful? 回答1: Valarrays (value arrays) are intended to bring some of the speed of Fortran to C++. You wouldn't make a valarray of pointers so the compiler can make assumptions about the code and optimise it better. (The main reason that Fortran is so fast is that there is no

问题 I have a custom iterator class conforming to the bidirectional iterator requirements (but not random access). However, the distance of two iterators can also be found in constant time. Conceptually, it2 - it1 is efficient, but it += n is not (neither of these operator overloads is actually implemented). Is it reasonable to overload std::distance() to allow standard library algorithms to compute distances efficiently with this iterator? I found conflicting information about the appropriateness

问题 So far, I'm able to get the current locale, but I want to get the date format for that particular locale. Can this be done with the standard library. #include <locale> int _tmain(int argc, _TCHAR* argv[]) { // Print the current locale std::cout << std::locale("").name().c_str() << "\n"; // TODO: get the locale's date pattern, example for US it's (mm/dd/yyyy) std::cout << "date pattern: \n"; } 回答1: If you just want to cast a date to the corresponding string you can use std::time_put<char> :

问题 For API/ABI compatibility across many toolchains with the same binary, it is well known that STL containers, std::string , and other standard library classes like iostreams are verboten in public headers. (Exceptions to this are if one is distributing one build for each version of supported toolchains; one delivers source with no binaries for end-user compilation, which are not preferred options in the present case; or one translates to some other container inline so that a differing std

问题 I was wondering why does the c++ standard require that std::sort should only take random-access iterators? I don't see the advantage, since both std::sort and std::list::sort have a complexity of N*log(N) . Restricting std::sort to random-access iterators (RAI) seems to have made it necessary to write a separate function for lists with the same complexity. The same applies to partial_sort , where the non-RAI counter-part for list is simply missing to this day. Is this design because people

问题 Consider this code: class A : public std::enable_shared_from_this<A> { public: std::shared_ptr<A> f() { return shared_from_this(); } }; int main() { A a; std::shared_ptr<A> ptr = a.f(); } This code terminated in Visual Studio 2017. I guess I am doing something wrong here. Can anyone help me with this? I want a shared_ptr on a created by shared_from_this(). 回答1: Because a is not a owned by a shared pointer. From cppreference: It is permitted to call shared_from_this only on a previously shared