c++-faq

问题 I posted a question with my code whose only #include directive was the following: #include <bits/stdc++.h> My teacher told me to do this, but in the comments section I was informed that I shouldn't. Why? 回答1: Including <bits/stdc++.h> appears to be an increasingly common thing to see on Stack Overflow, perhaps something newly added to a national curriculum in the current academic year. I imagine the advantages are vaguely given thus: You only need write one #include line You do not need to

问题 Quote from The C++ standard library: a tutorial and handbook: The only portable way of using templates at the moment is to implement them in header files by using inline functions. Why is this? (Clarification: header files are not the only portable solution. But they are the most convenient portable solution.) 回答1: Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer. Anyway, the reason your code is failing is that,

问题 What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them? Feel free to edit/add your own. 回答1: Compiling a C++ program takes place in several steps, as specified by 2.2 (credits to Keith Thompson for the reference): The precedence among the syntax rules of translation is specified by the following phases [see footnote] . Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set

问题 Suppose I want to make a new deduction guide making the following possible ? std::string str; std::basic_string_view sv = str; Would that be an Ok customization ? 回答1: [namespace.std]/2.4: The behavior of a C++ program is undefined if it declares [...] a deduction guide for any standard library class template. 来源： https://stackoverflow.com/questions/58992794/can-i-add-a-deduction-guide-to-std-namespace

问题 What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them? Feel free to edit/add your own. 回答1: Compiling a C++ program takes place in several steps, as specified by 2.2 (credits to Keith Thompson for the reference): The precedence among the syntax rules of translation is specified by the following phases [see footnote] . Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set

问题 Quote from The C++ standard library: a tutorial and handbook: The only portable way of using templates at the moment is to implement them in header files by using inline functions. Why is this? (Clarification: header files are not the only portable solution. But they are the most convenient portable solution.) 回答1: Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer. Anyway, the reason your code is failing is that,

问题 Quote from The C++ standard library: a tutorial and handbook: The only portable way of using templates at the moment is to implement them in header files by using inline functions. Why is this? (Clarification: header files are not the only portable solution. But they are the most convenient portable solution.) 回答1: Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer. Anyway, the reason your code is failing is that,

问题 I have written a simple, working tetris game with each block as an instance of a class singleblock. class SingleBlock { public: SingleBlock(int, int); ~SingleBlock(); int x; int y; SingleBlock *next; }; class MultiBlock { public: MultiBlock(int, int); SingleBlock *c, *d, *e, *f; }; SingleBlock::SingleBlock(int a, int b) { x = a; y = b; } SingleBlock::~SingleBlock() { x = 222; } MultiBlock::MultiBlock(int a, int b) { c = new SingleBlock (a,b); d = c->next = new SingleBlock (a+10,b); e = d-

问题 In templates, where and why do I have to put typename and template on dependent names? What exactly are dependent names anyway? I have the following code: template <typename T, typename Tail> // Tail will be a UnionNode too. struct UnionNode : public Tail { // ... template<typename U> struct inUnion { // Q: where to add typename/template here? typedef Tail::inUnion<U> dummy; }; template< > struct inUnion<T> { }; }; template <typename T> // For the last node Tn. struct UnionNode<T, void> { //

问题 I have the following piece of code that prompts the user for their name and state: #include <iostream> #include <string> int main() { std::string name; std::string state; if (std::cin >> name && std::getline(std::cin, state)) { std::cout << "Your name is " << name << " and you live in " << state; } } What I find is that the name has been successfully extracted, but not the state. Here is the input and resulting output: Input: "John" "New Hampshire" Output: "Your name is John and you live in "