map inserting keys but not value - c++

Question

I am playing with c++ code today. Learning about std containers. I'm trying to insert and update data in a std::map but for some reason I can't insert values into a map. Keys will insert but not values. The code at the bottom will print the following if you enter something into the terminal that opens. In this example I entered "test". Anyway, my questions are, why is the insert returning false, why in the value not inserting?
test
first
failed
Context1 :
Here is the code:
#include "stdafx.h"
#include <string>
#include <iostream>
#include <map>
#include <random>
static std::map<std::string, std::string> currentFullState;
static const std::string sDEFAULT_STRING = "";
void PringCurrentState()
{
std::map<std::string, std::string>::iterator stateData = currentFullState.begin();
while (stateData != currentFullState.end())
{
std::cout << stateData->first << " : ";
std::cout << stateData->second << std::endl;
stateData++;
};
}
void UpdateState(std::string context, std::string data)
{
if (currentFullState[context] == sDEFAULT_STRING)
{
// first entry, possibly special?
std::cout << "first" << std::endl;
auto result = currentFullState.insert(std::make_pair(context, data.c_str()));
if (result.second == false)
std::cout << "failed" << std::endl;
else
std::cout << "good" << std::endl;
}
else if (data != currentFullState[context])
{
// change in value
}
else
{
currentFullState[context] == data;
}
}
void DoWork()
{
if (rand() % 2)
{
UpdateState("Context1", "Data1");
}
else
{
UpdateState("Context2", "Data2");
}
}
int main()
{
std::string command = "";
for (;;)
{
PringCurrentState();
std::cin >> command;
DoWork();
if (command == "q")
{
break;
}
}
return 0;
}
Why does the insert not work?

Answer 1

Certainly would help if you wrote
currentFullState[context] = data;
instead of
currentFullState[context] == data;
Also
auto result = currentFullState.insert(std::make_pair(context, data));
should be preferred to
auto result = currentFullState.insert(std::make_pair(context, data.c_str()));
Slightly surprised that the second one compiles.
=========================================================================
The real reason the insert fails is that you are adding that key for the second time. This is the first time
if (currentFullState[context] == sDEFAULT_STRING)
operator[] on a map always adds the key to the map. This is why your second attempt to add with
auto result = currentFullState.insert(std::make_pair(context, data.c_str()));
fails, the key is already present. If you had written
currentFullState[context] = data;
Then it would work.