std

问题 When I access an element in std::unordered_map using operator [] for the first time, it is automatically created. What (if any) are guarantees about its initialization? (It is guaranteed to be value initialized, or only to be constructed)? Example: std::unordered_map<void *, size_t> size; char *test = new char[10]; size[test] += 10; Is size[test] guaranteed to be 10 at the end of this sequence? 回答1: Is size[test] guaranteed to be 10 at the end of this sequence? Yes. In the last line of your

问题 Today I'm trying to switch from VS2019 to VS Code while keep working with MSVC. This way I will develop in a lightweight and easier environment most of the time, and when I need advanced stuff such as seeing hot paths, I would be able to open VS2019 and do the job. Below is my launch.json configuration "name": "debug", "type": "cppvsdbg", "request": "launch", "program": "${workspaceFolder}/build/${fileBasenameNoExtension}.exe", "args": [], "stopAtEntry": false, "cwd": "${workspaceFolder}",

问题 I'm a cs student at the Technion, I have just learned of 'errno' variable and c-style func. call. This makes me wonder, if c-style syscall uses registers to return a value, why should anyone use 'errno' at all? 回答1: The main reason for using errno is to give more information about the error condition. This is especially useful in situations where most (or even all ) possible return values of a function are actually valid return values. Consider the fopen() function, which returns a pointer to

问题 I'm a cs student at the Technion, I have just learned of 'errno' variable and c-style func. call. This makes me wonder, if c-style syscall uses registers to return a value, why should anyone use 'errno' at all? 回答1: The main reason for using errno is to give more information about the error condition. This is especially useful in situations where most (or even all ) possible return values of a function are actually valid return values. Consider the fopen() function, which returns a pointer to

问题 This member function, that has no defined effect in the standard (only remarks), would have limited use if not allowed to reallocate. But the only paragraph I found in the standard that seems to apply would be 23.2.1/11: "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within

问题 This member function, that has no defined effect in the standard (only remarks), would have limited use if not allowed to reallocate. But the only paragraph I found in the standard that seems to apply would be 23.2.1/11: "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within

问题 I have the following function, which writes a vector to a CSV file: #include <math.h> #include <vector> #include <string> #include <fstream> #include <iostream> #include <iterator> using namespace std; bool save_vector(vector<double>* pdata, size_t length, const string& file_path) { ofstream os(file_path.c_str(), ios::binary | ios::out); if (!os.is_open()) { cout << "Failure!" << endl; return false; } os.precision(11); copy(pdata->begin(), pdata->end(), ostream_iterator<double>(os, ",")); os

问题 I am trying to create an std::set with a function I defined for sorting, but I get the error: "Error: function "GFX::MeshCompare" is not a type name" Mesh.h namespace GFX { struct Mesh { [...] }; inline bool MeshCompare(const Mesh& a, const Mesh& b) { return ( (a.pTech < b.pTech) || ( (b.pTech == a.pTech) && (a.pMaterial < b.pMaterial) ) || ( (b.pTech == a.pTech) && (a.pMaterial == b.pMaterial) && (a.topology < b.topology) ) ); } }; Renderer.h namespace GFX { class Renderer { private: [...]

问题 I am trying to create an std::set with a function I defined for sorting, but I get the error: "Error: function "GFX::MeshCompare" is not a type name" Mesh.h namespace GFX { struct Mesh { [...] }; inline bool MeshCompare(const Mesh& a, const Mesh& b) { return ( (a.pTech < b.pTech) || ( (b.pTech == a.pTech) && (a.pMaterial < b.pMaterial) ) || ( (b.pTech == a.pTech) && (a.pMaterial == b.pMaterial) && (a.topology < b.topology) ) ); } }; Renderer.h namespace GFX { class Renderer { private: [...]

问题 I am trying to create an std::set with a function I defined for sorting, but I get the error: "Error: function "GFX::MeshCompare" is not a type name" Mesh.h namespace GFX { struct Mesh { [...] }; inline bool MeshCompare(const Mesh& a, const Mesh& b) { return ( (a.pTech < b.pTech) || ( (b.pTech == a.pTech) && (a.pMaterial < b.pMaterial) ) || ( (b.pTech == a.pTech) && (a.pMaterial == b.pMaterial) && (a.topology < b.topology) ) ); } }; Renderer.h namespace GFX { class Renderer { private: [...]