container use after clear

[tehuser]

Sorry if this is a duplicate. I used the search to find out, but couldn't see any.

#include <map>
#include <iostream>

using namespace std;

int main()
{
	map<int, int> m;

	m[0]=2;
	m[1]=4;
	m[2]=-2;
	m[3]=-5;


	m.clear();

	cerr << m[2] << endl;
}

$gcc test.cpp -o test -Wall --pedantic -lstdc++
$cppcheck --enable=all test.cpp
Checking test.cpp...
$cppcheck --version
Cppcheck 1.48

[hyd_danmar]

Good suggestion, thanks! I am pretty sure there is no ticket about this.

It shouldn't be too hard to implement this.

[kimmov]

This is valid and defined behavior for the std::map.

Reading documentation always helps:
http://www.cplusplus.com/reference/stl/map/operator%5B%5D/

[x being the index to access]

If x does not match the key of any element in the container,
the function inserts a new element with that key and returns
a reference to its mapped value.

There is even an example code doing this.

[tehuser]

I don't think thats right. This is only true if you

map[value] = key;

and value does not exist in the map. Then it will insert the value, key pair into the map. This does not happen if you do

  something = map[value];

Or you could simply alter the example and use .at instead, which does not do insertion, or change the map to a vector.

[tehuser]

Sorry, more correct semantics:

map[key] = value;

will insert

and

newvalue = map[key];

wont insert.

[kimmov]

So you question that reference's correctness?

[tehuser]

I am questioning the relevance of this reference to this ticket -- it does not cover the case here. Simply alter my example to use "at" instead, if you wish -- this should be a completely unambiguous result. Or alternatively, alter the container.

here are some revised programs:

#include <vector>
#include <iostream>

using namespace std;

int main()
{
	vector< int> m;

	v.push_back(2);
	v.push_back(1);
	v.push_back(-2);
	v.push_back(-5);


	m.clear();

	cerr << m[2] << endl;
}

And also

#include <map>
#include <iostream>

using namespace std;

int main()
{
	map<int, int> m;

	m[0]=2;
	m[1]=4;
	m[2]=-2;
	m[3]=-5;


	m.clear();

	cerr << m.at(2) << endl;
}

[tehuser]

errata (actually using a compiler this time):

#include <vector>
#include <iostream>

using namespace std;

int main()
{
	vector< int> m;

	m.push_back(2);
	m.push_back(1);
	m.push_back(-2);
	m.push_back(-5);


	m.clear();

	cerr << m[2] << endl;
}