Get value from integer whose name is on a string

Question

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).

Answer 1

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:

• If you are willing to "encode" the 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.
• If you are willing to allow only single-character variables, you could replace the sequence of conditionals with a single switch.
• If you are open to using standard collections, replace the individual variables with a map of string to integer.

Answer 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;
}

Answer 3

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.