c++-standard-library

问题 This question already has answers here : C++98/03 std::is_constructible implementation (2 answers) Closed 2 years ago . So far I can't find anything ELI5 online. For a learning project I would like to implement my own is_constructible. Can someone explain how it works, please? 回答1: From cppreference: [I]f the variable definition T obj(std::declval<Args>()...); is well-formed, value is equal to true , else value is false . Checking whether code is well-formed can be done with SFINAE techniques

问题 I'm trying to copy a map into a vector of pair, so I can then sort the vector by the second data member of the pairs. I have resolved this doing like this: void mappedWordsListSorter(){ for (auto itr = mappedWordsList.begin(); itr != mappedWordsList.end(); ++itr){ vectorWordsList.push_back(*itr); } sort(vectorWordsList.begin(), vectorWordsList.end(), [=](pair<string, int>& a, pair<string, int>& b){return a.second > b.second;}); } I need to find a way to do this without using a raw loop, using

问题 Does the C++ standard library have an "ordered set" datastructure? By ordered set, I mean something that is exactly the same as the ordinary std::set but that remembers the order in which you added the items to it. If not, what is the best way to simulate one? I know you could do something like have a set of pairs with each pair storing the number it was added in and the actual value, but I dont want to jump through hoops if there is a simpler solution. 回答1: No single, homogeneous data

问题 I recently was bitten by the fact that ios_base::width and/or the setw manipulator have to be reset with every item written to the stream. That is, you must do this: while(whatever) { mystream << std::setw(2) << myval; } Rather than this: mystream.width(2); while(whatever) { mystream << myval; } Ok, fine. But does anyone know why this design decision was made? Is there some rationale that I'm missing, or is this just a dark corner of the standard? Other stream formatting modifiers (as

问题 According to cppreference.com, std::rel_ops::operator!=,>,<=,>= will be deprecated in C++20. What's the rationale behind? 回答1: In C++20, you get three-way comparison (operator <=> ), which automatically "generates" default comparisons if provided: struct A { // You only need to implement a single operator. std::strong_ordering operator<=>(const A&) const; }; // Compiler generates all 6 relational operators A to1, to2; if (to1 == to2) { /* ... */ } // ok if (to1 <= to2) { /* ... */ } // ok,

问题 Sample code: #include <iostream> #include <cmath> #include <stdint.h> using namespace std; static bool my_isnan(double val) { union { double f; uint64_t x; } u = { val }; return (u.x << 1) > 0x7ff0000000000000u; } int main() { cout << std::isinf(std::log(0.0)) << endl; cout << std::isnan(std::sqrt(-1.0)) << endl; cout << my_isnan(std::sqrt(-1.0)) << endl; cout << __isnan(std::sqrt(-1.0)) << endl; return 0; } Online compiler. With -ffast-math , that code prints "0, 0, 1, 1" -- without, it

问题 According to the C++ standard, are implementations of the C++ standard library allowed to strengthen noexcept specifications of methods and other functions of the C++ standard library as defined by the standard? For example, if the C++ standard specifies some function std::f as void f(); are standard library implementations allowed to implement it as void f() noexcept; instead? 回答1: The Standard says yes: § 17.6.5.12.1 Restrictions on exception handling [res.on.exception.handling] Any of the

问题 I'm trying to debug a program that has no source code available, and I need to look at what it has stored in a std::string. I've been Googling and looking on here, and I've found some information about outputting STL containers, but all of it refers to variables, with no source or debug information all I have is a memory offset of the class data. Is there any way to do this? 回答1: Every std::string implementation has a pointer to the raw characters in it somewhere. For g++ 4.x , that pointer

问题 I need a "string pool" object into which I can repeatedly insert a "sequence of chars" (I use this phrase to mean "string" without confusing it with std::string or a C string), obtain a pointer to the sequence, and be guaranteed that the pointer will not become invalidated if/when the pool needs to grow. Using a simple std::string as the pool won't work, because of the possibility for the string to be reallocated when it outgrows its initial capacity, thus invalidating all previous pointers

问题 I am playing with android ndk. I am using Window Vista with cygwin (latest version). I compiled and launched the hello world jni sample on my phone. It is working. The code is (is a .cpp file): #include <string.h> #include <jni.h> extern "C" { JNIEXPORT jstring JNICALL Java_org_android_helloworld_HelloworldActivity_invokeNativeFunction(JNIEnv* env, jobject javaThis); }; jstring Java_org_android_helloworld_HelloworldActivity_invokeNativeFunction(JNIEnv* env, jobject javaThis) { return env-