Reading from a map using indexing.

Hello,

I would like to validate one of my approach.
I am creating a map like

std::map<unsigned int, double> key_value_pair;

I insert the value like as follows

1 25.0
0 30.0
5 90.0
2 22.0

Then i would like to read the value like as follows

double value = key_value_pair[0];

I am able to compile & there is no error.

question is does this will give 30.0 always.?

I am using VS2005.

Please help.

-Dave

The following code

Code:

---

#include <iostream>
#include <map>
using namespace std;

typedef pair<unsigned int, double> uidpair;
typedef map<unsigned int, double> uidmap;

int main()
{
uidmap kvp;

        kvp.insert(uidpair(1, 25.0) );
        kvp.insert(uidpair(0, 30.0) );
        kvp.insert(uidpair(5, 90.0) );
        kvp.insert(uidpair(2, 22.0) );

        cout << 0 << " " << kvp[0] << endl;
        cout << 1 << " " << kvp[1] << endl;
        cout << 2 << " " << kvp[2] << endl;
        cout << 5 << " " << kvp[5] << endl;

        return 0;
}

---

always produces the output

Code:

---

0 30
1 25
2 22
5 90

---

Note that when accessing the map via the operator [] and the requested key does not match an element in the map, then the element is inserted using its default constructor.

Hi

I am satisfy with the answer. But this approach is worth when using VS compiler.
When i used this approach in 'Release' mode in VS 2005 a random failure is detected.(A null value returned)
I ensured the key is valid always with a proper value.

One more info from my issue :-
Key is of type unsigned int and value is of type std::string.

Code:

---

std::map<unsigned int, std::string> key_value_pair;
//! Reference :- http://www.cplusplus.com/reference/map/map/operator%5B%5D/

---

Any restriction when using std::string with map, like a null character ?

Any guess ...

-Dave

There is no such thing as a null std::string. Empty yes, null, no.

The code may compile, but storing nulls in a std::string is undefined behaviour.

Regards,

Paul McKenzie

Quote:

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Originally Posted by Paul McKenzie

The code may compile, but storing nulls in a std::string is undefined behaviour.

---

I don't think so: std::string is not a null terminated string type, so there is nothing special about null characters with respect to std::string. Hence, I see no reason why storing a null character in a std::string will result in undefined behaviour.

Quote:

---

When i used this approach in 'Release' mode in VS 2005 a random failure is detected.(A null value returned)
I ensured the key is valid always with a proper value.

---

What do you mean by a 'random' failure? Are you using map in multi-threaded code? Can you post a simple example that demonstrates the issue? Altering my simple test program in post #2 to have value of type std::string always produces the expected output on my computer.

Quote:

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Originally Posted by laserlight

I don't think so: std::string is not a null terminated string type, so there is nothing special about null characters with respect to std::string. Hence, I see no reason why storing a null character in a std::string will result in undefined behaviour.

---

Maybe I should clarify.

There is a difference between storing null characters in a std::string, and assigning a null to a std::string object.

Code:

---

std::string s = NULL;

---

This is undefined behaviour. The reason is that the conversion rules for NULL will cause std::string to believe it is being assigned a pointer to char. Once that happens, all bets are off.

Regards,

Paul McKenzie

On my MSVC, the following code

Code:

---

#include "string"
#include "iostream"
using namespace std;

int main()
{
string asd = NULL;

    cout << "asd: '" << asd << "'" << endl;
}

---

compiles but produces a run-time error

Quote:

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The instruction at "0x00404d90" referenced memory at "0x00000000". The memory could not be "read".

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for the asd definiton statement.