Removing elements from a container or asking if an associative container has a specific key, is too complicated. I should say was because with C++20 the story changes.

Let me start simple. You want to erase an element from a container.
The erase-remove Idiom
Okay. Removing an element from a container is quite easy. In case of a std::vecto
r you can use the function std::remove.
// removeElements.cpp
#include <algorithm>
#include <iostream>
#include <vector>
int main() {
std::cout << std::endl;
std::vector myVec{-2, 3, -5, 10, 3, 0, -5 };
for (auto ele: myVec) std::cout << ele << " ";
std::cout << "\n\n";
std::remove_if(myVec.begin(), myVec.end(), [](int ele){ return ele < 0; }); // (1)
for (auto ele: myVec) std::cout << ele << " ";
std::cout << "\n\n";
}
The program removeElemtens.cpp
removes all elements from the std::vector
that are smaller than zero. Easy, or? Now, you fall into the trap that is well-known to each professional C++ programmer.

std::remove
or std::remove_if
in line (1) does not remove anything. The std::vector
still has the same number of arguments. Both algorithms return the new logical end of the modified container.
To modify a container, you have to apply the new logical end to the container.
// eraseRemoveElements.cpp
#include <algorithm>
#include <iostream>
#include <vector>
int main() {
std::cout << std::endl;
std::vector myVec{-2, 3, -5, 10, 3, 0, -5 };
for (auto ele: myVec) std::cout << ele << " ";
std::cout << "\n\n";
auto newEnd = std::remove_if(myVec.begin(), myVec.end(), // (1)
[](int ele){ return ele < 0; });
myVec.erase(newEnd, myVec.end()); // (2)
// myVec.erase(std::remove_if(myVec.begin(), myVec.end(), // (3)
[](int ele){ return ele < 0; }), myVec.end());
for (auto ele: myVec) std::cout << ele << " ";
std::cout << "\n\n";
}
Line (1) returns the new logical end newEnd
of the container myVec
. This new logical end is applied in the line (2) to remove all elements from myVec
starting at newEnd
. When you apply the functions remove and erase in one expression such as in line (3), you exactly see, why this construct is called erase-remove-idiom.

Thanks to the new functions erase
and erase_if
in C++20, erasing elements from containers is way more convenient.
erase
and erase_if
in C++20
With erase
and erase_if
, you can directly operate on the container. In contrast, the previous presented erase-remove idiom is quite verbose (line 3 in eraseRemoveElements.cpp
): erase
requires two iterators which I provided by the algorithm std::remove_if
.
Let's see what the new functions erase
and erase_if
mean in practice. The following program erases elements for a few containers.
// eraseCpp20.cpp
#include <iostream>
#include <numeric>
#include <deque>
#include <list>
#include <string>
#include <vector>
template <typename Cont> // (7)
void eraseVal(Cont& cont, int val) {
std::erase(cont, val);
}
template <typename Cont, typename Pred> // (8)
void erasePredicate(Cont& cont, Pred pred) {
std::erase_if(cont, pred);
}
template <typename Cont>
void printContainer(Cont& cont) {
for (auto c: cont) std::cout << c << " ";
std::cout << std::endl;
}
template <typename Cont> // (6)
void doAll(Cont& cont) {
printContainer(cont);
eraseVal(cont, 5);
printContainer(cont);
erasePredicate(cont, [](auto i) { return i >= 3; } );
printContainer(cont);
}
int main() {
std::cout << std::endl;
std::string str{"A Sentence with an E."};
std::cout << "str: " << str << std::endl;
std::erase(str, 'e'); // (1)
std::cout << "str: " << str << std::endl;
std::erase_if( str, [](char c){ return std::isupper(c); }); // (2)
std::cout << "str: " << str << std::endl;
std::cout << "\nstd::vector " << std::endl;
std::vector vec{1, 2, 3, 4, 5, 6, 7, 8, 9}; // (3)
doAll(vec);
std::cout << "\nstd::deque " << std::endl;
std::deque deq{1, 2, 3, 4, 5, 6, 7, 8, 9}; // (4)
doAll(deq);
std::cout << "\nstd::list" << std::endl;
std::list lst{1, 2, 3, 4, 5, 6, 7, 8, 9}; // (5)
doAll(lst);
}
Line (1) erases all character e
from the given string str.
Line (2) applies the lambda expression to the same string and erases all upper case letters.
In the remaining program, elements of the sequence containers std::vecto
r (line 3), std::deque
(line 4), and std::list
(line 5) are erased. On each container, the function template doAll
(line 6) is applied. doAll
erases the element 5 and all elements greater than 3. The function template erase
(line 7) uses the new function erase
and the function template erasePredicate
(line 8) uses the new function erase_if
.
Thanks to the Microsoft Compiler, here it the output of the program.

The new functions erase
and erase_if
can be applied to all containers of the Standard Template Library. This does not hold for the next convenience function contains
.
Checking the Existence of an Element in an Associative Container
Thanks to the functions contains
, you can easily check if an element exists in an associative container.
Stopp, you may say, we can already do this with find or count.
No, both functions are not beginners friendly and have their downsides.
// checkExistens.cpp
#include <set>
#include <iostream>
int main() {
std::cout << std::endl;
std::set mySet{3, 2, 1};
if (mySet.find(2) != mySet.end()) { // (1)
std::cout << "2 inside" << std::endl;
}
std::multiset myMultiSet{3, 2, 1, 2};
if (myMultiSet.count(2)) { // (2)
std::cout << "2 inside" << std::endl;
}
std::cout << std::endl;
}
The functions produce the expected result.

Here are the issues with both calls. The find
call in line (1) is too verbose. The same argumentation holds for the count
call in line (2). The count
call has also a performance issue. When you want to know if an element is in a container, you should stop when you found it and not count until the end. In the concrete case myMultiSet.count(2)
returned 2.
On the contrary, the contains member function in C++20 is quite convenient to use.
// containsElement.cpp
#include <iostream>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
template <typename AssozCont>
bool containsElement5(const AssozCont& assozCont) { // (1)
return assozCont.contains(5);
}
int main() {
std::cout << std::boolalpha;
std::cout << std::endl;
std::set<int> mySet{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
std::cout << "containsElement5(mySet): " << containsElement5(mySet);
std::cout << std::endl;
std::unordered_set<int> myUnordSet{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
std::cout << "containsElement5(myUnordSet): " << containsElement5(myUnordSet);
std::cout << std::endl;
std::map<int, std::string> myMap{ {1, "red"}, {2, "blue"}, {3, "green"} };
std::cout << "containsElement5(myMap): " << containsElement5(myMap);
std::cout << std::endl;
std::unordered_map<int, std::string> myUnordMap{ {1, "red"}, {2, "blue"}, {3, "green"} };
std::cout << "containsElement5(myUnordMap): " << containsElement5(myUnordMap);
std::cout << std::endl;
}
There is not much to add to this example. The function template containsElement5
returns true
if the associative container contains the key 5. In my example, I only used the associative containers std::set
, std::unordered_set
, std::map
, and std::unordered_set
which can not have a key more than once.

What's next?
The convenience functions go on in my next post. With C++20, you can calculate the midpoint of two values, check if a std::string
start or ends with a substring, and create callables with std::bind_front
.
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Comments
looks like the container is not in a well defined state.
remove_if does not realy remove the matched elements but moves them to the end of the container and returns a new end iterator which is pointing to the first removed element. The container is reordered but in a well defined state.
This is can be more efficient than erasing because no heap operations are involved.
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