typedef vector<int> vec; struct classcomp { bool operator() (const vec& vec1, const vec& vec2) const{ if (vec1[0] > vec2[0]){ return true; } return false; } }; int main() { set<vec,classcomp> v; for (int i = 0; i < 20; i++){ v.insert({rand()%20,i}); } cout << "size: " << v.size() << endl; for (auto e : v){… Read More Even though I inserted 20 (distinct) elements into my std::set, it only has a size of 14?

I created a set of pointers, and tried to erase one member in the following code: #include <iostream> #include <set> using namespace std; struct Node{ Node(int val){ this->val = val; } int val; Node* prev; Node* next; }; struct comp{ bool operator()(const Node* a, const Node* b) const{ return a->val<=b->val; } }; int main() {… Read More C++ erase from set of pointers does not work?

I have a json array as shown below { { "Data":[1.0, 2.0, 3.0, 4.0] } } I am trying to copy this json array to a float array using std::transform. In code – #include <nlohmann/json.hpp> #include <memory> using json = nlohmann::json; int main(){ json tensor; tensor["Data"] = { 1, 0, 2 }; auto dataPtr =… Read More std::transform in copying data from json array to a float array C++

I am trying to implement a transparency in my OpenGL prorgam. Currently I have such function (minimal reproducible example): #include <map> #include <unordered_map> #include <iostream> #include <vector> class Sprite { public: std::vector<int> m_meshes; std::pair<float, float> pos; }; int main() { std::unordered_map<int, Sprite> m_sprites = { { 0, { { 43 } } }, { 1,… Read More Why are the elements of the std::multimap changing after exiting the loop in this code?

is there nodiscard attribute available in c++11? If not, why no warning message output by g++ code belows. //test.cpp #include<iostream> [[nodiscard]] int foo() { return 1; } int main() { std::cout<<foo()<<std::endl; return 0; } At the beginning, i complie the program by "g++ -std=c++20 test.cpp -o test.o" and everythings is fine. But then i want… Read More No warning message generated by g++ when using [[nodiscard]] and compile with lower stl option

I’m reading the std::map part of The C++ Standard Library by Nicolai M. Josuttis. Here is a simplified code which can show the question bother me a lot: #include <iostream> #include <map> #include <string> class MyCompare { public: bool operator() (const std::string& lhs, const std::string& rhs) const { return lhs < rhs; } }; int… Read More why can I use a class as custom comparison criterion as a template argument to std::map without providing a class instance

Trying to sort an vector<pair<int,int>> decreasing order in C++. sort(v.begin(),v.end(),greater<int> ()) >Solution : The elements of your vector are pair<int,int>, so you should use greater<pair<int,int>> () instead of greater<int> ().

I’m learning std::set . Here is my codes: #include <set> #include <iostream> using namespace std; struct cmp{ bool operator () (const int & a,const int & b) const { if(abs(a-b)<=3) return false; return a < b; } }; set<int,cmp> q{1, 2, 10}; int main(){ if(q.find(4)!=q.end()) cout << 1; else cout << 2; } Output: 1… Read More c++ Overloaded operator () change std::set.find()

I want to create a dynamic memory for a car dealership for bikes and cars but the vector didn’t print the right things and has lots of repetitions that aren’t supposed to be there. I created an object for the overlapping variables for both vehicles so I can pass it through the bike class later… Read More Return the elements in an array using operator overloading with a class passing through it