std::map Why it is not adding key , values in my code - c++

Question

I am trying to add key & value data to my class map member variable - but it does not adds the same -I tried map - insert, [] and emplace method but they are not adding data to map in my loop code - only the value I have inserted during my class construction is available in it - what I need to do to fix the issue - I was expecting that the show method will also print 7, 8,9, 9 -
#include <iostream>
#include <map>
#include <vector>
class A {
public:
A(std::initializer_list <uint32_t> d): data(d) {}
std::vector <uint32_t> data;
bool operator < (const A & rhs) const {
size_t rhsSize = rhs.data.size();
for (size_t i = 0; i < data.size(); i++) {
if (i < rhsSize)
return false;
return true;
}
}
};
class B {
public:
B(const std::map <A, uint32_t> & t): table(t) {}
void Show() {
for (std::map <A, uint32_t> ::iterator it = table.begin(); it != table.end(); ++it) {
for (const auto & i: it->first.data)
std::cout << i << "\n";
std::cout << it->second << "\n";
}
}
std::map <A, uint32_t> table;
};
int main() {
std::map <A, uint32_t> tb = {
{
A {70, 8, 9,10}, 1234}
};
B b(tb);
for (int i = 0; i < 2; i++) {
b.Show();
b.table.emplace(A {7, 8,9, 9}, 1234);
}
return 0;
}
Compile and run the code:
$ c++ -std=c++11 try78.cpp
$ ./a.exe
70
8
9
10
1234
70
8
9
10
1234

Answer 1

Your operator < violates the strict weak ordering requirement that std::map requires of the key. That requires that if comp(a,b) is true then comp(b,a) is false. You use
bool operator < (const A & rhs) const {
size_t rhsSize = rhs.data.size();
for (size_t i = 0; i < data.size(); i++) {
if (i < rhsSize)
return false;
return true;
}
}
to compare the elements and if we compare {70, 8, 9,10} against {7, 8,9, 9} then it returns true and if we flip it around it also returns true. This makes the map think the elements are equal and it won't add the second item.
If you just to make sure that unique vectors are stored in the map then you can use std::vector's operator < in A's operator < like
bool operator < (const A & rhs) const {
return data < rhs.data;
}
and the code will function properly.