C++ :: Existing Code For String To Expression?

I would like to get hold of the code for a function that takes a string as input argument and returns a numerical value, either integer or double, according to the the expression represented by the string. For example the string may look like this:

Code:
"(256 * d3 + d4) * factor - offset"

then the function would calculate the value of

Code:
(256 * d[3] + d[4]) * factor - offset

where d is an integer array, factor and offset are double or int variables or constants. For my application the name for the array can be fixed to be "d", and the names for other variables and constants are known. The closest thing I could find on-line is this:

[URL]

which only does literal integer calculations, no double, no variables or constants. Are there existing code that does what I need or do I have to write my own?