std::map problem

[mwilliamson]

Sorry if I'm getting anoying, I can't std::map to work right.
Heres my code:

std::map<std::string,std::string> m_map;
...
m_map[key] = value;
...
std::string value(m_map[key]);

The last line generates the error:
error C2678: binary '[' : no operator defined which takes a left-hand operand of type 'const class std::map<class std::basic_string<char,struct std::char_traits<ch...

From what I remember, and see on the internet this is how map is supposed to work.. so why doesn't it? I have tried everything (except looping through ) to get my values back from map.

[DanM]

If you want to figure out an element with a specified key is present in the map, you should use map::find, not map:perator[]. map:perator[] has a small problem:

If the argument key value is not found, then it is inserted along with the default value of the data type.

http://msdn.microsoft.com/library/de...lrfMapfind.asp

http://msdn.microsoft.com/library/de...lrfMapoperator[].asp


Anyway, coming back to your compiling problem: can you post the exact C++ code and especially the value & key variable declaration ? I don't believe all the lines are in the same method because you use value in thes second line and then you declare it again in the last line.

Dan

[mwilliamson]

more or less:

class MapClass
{
private:
std::map<std::string,std::string> m_map;
protected:
std::string GetValue(const std::string key) const
{
return m_map&#091;key&#093;;
}
void SetValue(const std::string key.const std::string value)
{
m_map[key] = value;
}
}

I know that the operator will create a new element if none exists, but the way my program is working there will be more sets than gets. And if it tries to get a key that doesn't exist, I am ok with returning "".

Thanks for your help!

[DanM]

And where is the last line:

std::string value(m_map[key]);

?

Most probaly you are passing a map const reference which is not OK when using it with the [] operator. Look at the error message:

const class std::map

Dan

[mwilliamson]

no, thats the exact code that generates the error.

I tried

std::string MapClass::GetValue(const std::string key) const
{
  std::string value(m_map[key]);
  return value;
}

To get rid of the error, but both are the same.

From your suggestion, I removed the const return value from GetValue, and the error is gone. Why though?

std::string MapClass::GetValue(const std::string key)
{
  return m_map[key];
}

The above code compiles fine.

[DanM]

Your method is const. But the map:perator[] is not const and can modify the m_varmap variable !
Now, either get rid of the const thing or make m_varmap mutable.

Dan

[DanM]

Just compiled your code with the following change:

1) Making the MapClass::GetValue non const

or

2) Keeping MapClass::GetValue const and making m_varmap mutable

and it compiles without any errors

Dan