C erase from vector while iterating
If it is iterating when deleting elements, you must avoid using invalid iterators. ① The correct way is to use it = vec.erase(it), and use the valid iterator returned by erase to continue traversing; ② Recommended "erase-remove" idiom for batch deletion: vec.erase(std::remove_if(vec.begin(), vec.end(), condition), vec.end()), which is safe and efficient; ③ You can use a reverse iterator to delete from back to front, the logic is clear, but you need to pay attention to the direction of the condition. Conclusion: Always update the iterator with the erase return value, prohibiting operations on the failed iterator, otherwise undefined behavior will result.
In C, if you are iterating when deleting an element from std::vector , you need to be careful, because erase() will invalidate all iterators after the deleted element . If handled improperly, undefined behavior (such as crashing or skipping elements).
But correctly using the return value of erase() can safely delete elements while traversing.
✅ Correct way: Use the iterator returned by erase()
std::vector::erase() returns an iterator pointing to the elements after the deleted element . We can take advantage of this feature to avoid using failed iterators.
#include <vector>
#include <iostream>
std::vector<int> vec = {1, 2, 3, 4, 5};
for (auto it = vec.begin(); it != vec.end(); ) {
if (*it == 3) {
it = vec.erase(it); // erase returns the next valid iterator} else {
it;
}
}✅ This is a standard, safe and recommended approach.
❌ Error example: Iterator is still incremented after deletion
for (auto it = vec.begin(); it != vec.end(); it) {
if (*it == 3) {
vec.erase(it); // Error! it expires, and then undefined behavior}
}This can cause program crashes or unpredictable behavior.
? A simpler way: use remove_if erase (recommended for conditional deletion)
If you want to delete multiple elements according to conditions, it is recommended to use the "erase-remove" idiom :
vec.erase(
std::remove_if(vec.begin(), vec.end(), [](int n) {
return n == 3; // Delete all elements equal to 3}),
vec.end()
);✅ Efficient, safe, clear code, suitable for scenarios where complex logic is not required to be executed when deleting.
? Summary: Comparison of three common solutions
| method | Applicable scenarios | Recommended | Things to note |
|---|---|---|---|
erase() returns the iterator | When deleting, you need to judge complex conditions or perform other operations. | ✅ Recommended | The return value must be received correctly, and do not Deleted it |
erase-remove idiom | Simple condition batch deletion | ✅ Highly recommended | The code is simpler and the performance is better |
Reverse iterator rbegin/rend | Delete from behind to front, some logic is more intuitive | ✅ Available | Pay attention to the logical direction of the condition |
? Tips: Use reverse iterators to avoid interference
If you want to delete from behind, you can also use a reverse iterator to avoid affecting the previous index:
for (auto it = vec.rbegin(); it != vec.rend(); ) {
if (*it == 3) {
it = vec.erase(it); // erase also returns the next one on reverse_iterator} else {
it;
}
} Note: erase behavior on reverse_iterator is normal, but be careful whether the logic needs reverse processing.
✅ Conclusion
- Iterate when deletion : use
it = vec.erase(it), don't - Batch conditional removal : Use
remove_if erasefirst. - Avoid using
itaftererase, otherwise it is undefined behavior.
Basically all is it, not complicated but it is easy to ignore details.
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