std

问题 It seems the std::hash functions for the C++17 string_view are not constexpr's. It seems to me that a string view bound to a const char[] could be hashed at compile time (which would be very sweet), or is there anything which prevents this? 回答1: Since C++14 (see 17.6.3.4 Hash requirements, table 26), we have: The value returned shall depend only on the argument k for the duration of the program. [Note: Thus all evaluations of the expression h(k) with the same value for k yield the same result

问题 It seems the std::hash functions for the C++17 string_view are not constexpr's. It seems to me that a string view bound to a const char[] could be hashed at compile time (which would be very sweet), or is there anything which prevents this? 回答1: Since C++14 (see 17.6.3.4 Hash requirements, table 26), we have: The value returned shall depend only on the argument k for the duration of the program. [Note: Thus all evaluations of the expression h(k) with the same value for k yield the same result

问题 Can anyone explain the logic behind the working of defined function in this program. This gives the correct output in this case even after commenting the whole logic of function. #include <iostream> using namespace std; void swap(int *a, int *b) { /* int temp=0; temp= *a; *a= *b; *b=temp; */ } int main() { int x, y; cout << "This program is the demo of function call by pointer \n\n"; cout << "Enter the value of x & y \n"; cout << "x: "; cin >> x; cout << "y: "; cin >> y; cout << "Value befor

问题 Can anyone explain the logic behind the working of defined function in this program. This gives the correct output in this case even after commenting the whole logic of function. #include <iostream> using namespace std; void swap(int *a, int *b) { /* int temp=0; temp= *a; *a= *b; *b=temp; */ } int main() { int x, y; cout << "This program is the demo of function call by pointer \n\n"; cout << "Enter the value of x & y \n"; cout << "x: "; cin >> x; cout << "y: "; cin >> y; cout << "Value befor

问题 While playing around with std::variant and std::visit the following question came up: Consider the following code: using Variant = std::variant<int, float, double>; auto lambda = [](auto&& variant) { std::visit( [](auto&& arg) { using T = std::decay_t<decltype(arg)>; if constexpr (std::is_same_v<T, int>) { std::cout << "int\n"; } else if (std::is_same_v<T, float>) { std::cout << "float\n"; } else { std::cout << "double\n"; } }, variant); }; It works fine as the following examples show: lambda

问题 I tried to compile the following program with different compilers (including gcc 6.1) : #include <optional> int main() { std::optional<int> o1; } Output is main.cpp:1:20: fatal error: optional: No such file or directory #include optional This is even true for the examples given here: http://en.cppreference.com/w/cpp/utility/optional/optional Any clues why? 回答1: std::optional will be part of the C++17 standard, but if you want to use before then you will have to use std::experimental::optional

问题 Now that Android Studio 2.2 is released officially, I'm migrating from my old ndk-build process to try and use CMake within AS. As I'm incorporating several codebases from within my company (that I can't edit) that make heavy use of C++11 code (including the dreaded std::to_string() method), the only way I can compile is with a select few configuration options - all of which I discovered earlier when beginning work with ndk-build. (see below) So everything compiles again and builds into the

问题 I'm making a little utility to help me remember passwords by repetition. I'd like to enter password to be remembered only once every day and not before each session. Of course, I wouldn't store a password itself, but would gladly store its hash. So, what are the easiest ways to get a hash from std::string using the C++ Standard Library? 回答1: For a quick solution involving no external libraries, you can use hash<std::string> to hash string s. It's defined by including the header files hash_map

问题 Hereafter, we use N4140 (C++14 Standard). According to § 17.6.3.4 Hash requirements , The value returned shall depend only on the argument k for the duration of the program . [ Note: Thus all evaluations of the expression h(k) with the same value for k yield the same result for a given execution of the program . — end note ] and § 20.9.12 Class template hash says ... the instantiation hash<Key> shall: (1.1) — satisfy the Hash requirements (17.6.3.4) ... (1.2) — ... This means a hash value of

问题 This std::regex line("[\s]+\+?[0-9]+.[0-9]+[\s]+\+?[0-9]+.[0-9]+[\s]+\+?[0-9]+.[0-9]+[\s]"); line causes this Exception thrown at 0x00007FFE39E69E08 in DosyaOkuHizli.exe: Microsoft C++ exception: std::regex_error at memory location 0x000000F751EFEAB0. Exception thrown at 0x00007FFE39E69E08 in DosyaOkuHizli.exe: Microsoft C++ exception: [rethrow] at memory location 0x0000000000000000. Exception thrown at 0x00007FFE39E69E08 in DosyaOkuHizli.exe: Microsoft C++ exception: std::regex_error at