int a = 1;
int b = 2;
int c = 3;
std::string TakeFrom = "a";
How can I get the value from TakeFrom, in this case, a, and read the integer named like that? For example, in the code above I would have to read a's value because 'a' is what TakeFrom contains, so I'd get "1", but if TakeFrom contained "b", I'd get b's value (2).
In normal C/C++ conditions, variable names only exist for their given scope during compilation, they are essentially human-readable mnemonics for referencing specific memory locations/registers.
C/C++ has no-to-minimal reflection capabilities, so there is no built-in way, at runtime, to reference a variable by name or look up the name of a variable by it's location/value.
To solve the problem you are describing, there are several simple options:
int value;
if (TakeFrom == "a") {
value = a;
} else if (TakeFrom == "b") {
value = b;
} ...
or
switch (TakeFrom[0]) {
case 'a':
value = a;
break;
case 'b':
value = b;
break;
}
or you could use a lookup table
int a = 1, b = 2, c = 3;
struct Lookup {
int& m_variable;
const char* m_name;
};
static const size_t NumLookups = 3;
Lookup lookups[NumLookups] = { { a, "a" }, { b, "b" }, { c, "c" } };
for (size_t i = 0; i < NumLookups; ++i) {
if (TakeFrom == lookups[i].m_name)
value = lookups[i].m_variable;
}
If a, b, and c must stay the local ints, the only way you do it is the boring one, i.e. like this:
int val;
if (TakeFrom == "a") {
val = a;
} else if (TakeFrom == "b") {
val = b;
} else if (TakeFrom == "c") {
val = c;
} else {
// It's an error
}
If you are open to some changes, there are several things that you could do:
TakeFrom, making it an integer instead of a string (say, 0 means a, 1 means b, and 2 means c), you could place a, b, and c into an array, and use the integer TakeFrom as an index.switch.
With there separate variables, there's no easy way (other than a series of if statements or equivalent).
If you turned the three variables into a single std::map<std::string,int>, things would be a lot easier.
std::mapto map strings (variable names) to ints.