As @Pete Becker points out in the comments, you need to qualify the name string as std::string:
private:
std::string constValue;
std::string varName;
The compiler just doesn’t know what you’re talking about, and it’s the equivalent of just writing:
SomeGreatType myMagicalUniversalType
The compiler just doesn’t know what type that is unless you’ve declared, hence the error
missing type specifier – int assumed
You should read up about why you should avoid using namespace std;.
With regards to your question in the comments:
In all the (working) classes I’ve written, I’ve never put in the std::, instead relying on the using namespace std; in the .cpp file. Why would this one be any different?
I can only infer that at some point before including “Parameter.h” that you had a using namespace std. E.g.:
// SomeType.h
#using namespace std
…
// Parameter.cpp
#include “SomeType.h”
#include “Parameter.h”
The compiler compiles things top-to-bottom, and including essentially just replaces the #include with the contents of that file