Debug Iterator Support
The Visual C++ run-time library detects incorrect iterator use, and asserts and displays a dialog box at run time. To enable debug iterator support, you must use a debug version of a C run-time library to compile your program. For more information, see CRT Library Features. For information about how to use iterators, see Checked Iterators.
The C++ standard describes how member functions might cause iterators to a container to become invalid. Two examples are:
Erasing an element from a container causes iterators to the element to become invalid.
Increasing the size of a vector (push or insert) causes iterators into the vector to become invalid.
Example
If you compile the following program in debug mode, at run time it will assert and terminate.
/* compile with /EHsc /MDd */
#include <vector>
#include <iostream>
int main() {
std::vector<int> v ;
v.push_back(10);
v.push_back(15);
v.push_back(20);
std::vector<int>::iterator i = v.begin();
++i;
std::vector<int>::iterator j = v.end();
--j;
std::cout<<*j<<'\n';
v.insert(i,25);
std::cout<<*j<<'\n'; // Using an old iterator after an insert
}
You can use the symbol _HAS_ITERATOR_DEBUGGING to turn off the iterator debugging feature in a debug build. The following program does not assert, but still triggers undefined behavior.
Important
Use _ITERATOR_DEBUG_LEVEL to control _HAS_ITERATOR_DEBUGGING. For more information, see _ITERATOR_DEBUG_LEVEL.
// iterator_debugging.cpp
// compile with: /EHsc /MDd
#define _HAS_ITERATOR_DEBUGGING 0
#include <vector>
#include <iostream>
int main() {
std::vector<int> v ;
v.push_back(10);
v.push_back(15);
v.push_back(20);
std::vector<int>::iterator i = v.begin();
++i;
std::vector<int>::iterator j = v.end();
--j;
std::cout<<*j<<'\n';
v.insert(i,25);
std::cout<<*j<<'\n'; // Using an old iterator after an insert
}
20 -572662307
An assert also occurs if you attempt to use an iterator before it is initialized, as shown here:
/* compile with /EHsc /MDd */
#include <string>
using namespace std;
int main() {
string::iterator i1, i2;
if (i1 == i2)
;
}
The following code example causes an assertion because the two iterators to the for_each algorithm are incompatible. Algorithms check to determine whether the iterators that are supplied to them are referencing the same container.
/* compile with /EHsc /MDd */
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
vector<int> v1;
vector<int> v2;
v1.push_back(10);
v1.push_back(20);
v2.push_back(10);
v2.push_back(20);
// The next line will assert because v1 and v2 are
// incompatible.
for_each(v1.begin(), v2.end(), [] (int& elem) { elem *= 2; } );
}
Notice that this example uses the lambda expression [] (int& elem) { elem *= 2; } instead of a functor. Although this choice has no bearing on the assert failure—a similar functor would cause the same failure—lambdas are a very useful way to accomplish compact function object tasks. For more information about lambda expressions, see Lambda Expressions in C++.
Debug iterator checking also causes an iterator variable that's declared in a for loop to be out of scope when the for loop scope ends.
// debug_iterator.cpp
// compile with: /EHsc /MDd
#include <vector>
#include <iostream>
int main() {
std::vector<int> v ;
v.push_back(10);
v.push_back(15);
v.push_back(20);
for (std::vector<int>::iterator i = v.begin() ; i != v.end(); ++i)
;
--i; // C2065
}
Debug iterators have non-trivial destructors. If a destructor does not run, for whatever reason, access violations and data corruption might occur. Consider this example:
/* compile with: /EHsc /MDd */
#include <vector>
struct base {
// FIX: uncomment the next line
// virtual ~base() {}
};
struct derived : base {
std::vector<int>::iterator m_iter;
derived( std::vector<int>::iterator iter ) : m_iter( iter ) {}
~derived() {}
};
int main() {
std::vector<int> vect( 10 );
base * pb = new derived( vect.begin() );
delete pb; // doesn't call ~derived()
// access violation
}