Is it really impossible to make a string into a variable in c++

Question

Both Convert string to variable name or variable type and How to use a string as a variable name in C++? answers say you can not use a string as a variable after compile time. However, they mention that higher level languages such as python can. As I understand it, Python is written in C from this https://softwareengineering.stackexchange.com/questions/20988/why-is-python-written-in-c-and-not-in-c .

So if python is written in C, and it can use string as variable, then why cant people do this in C++.

I guess the question is not clear. From the analogies in comments,

If Python is made from C, I would assume there is some function written that allows Python to be dynamic. So why cant that functionality be written without all of the other parts of Python.

Answer 1

When a C++ program is compiled, the variables names aren't put into the resulting executable. Once the compiler has everything figured out, they are literally discarded. All that is left behind are the very low level pieces of assembly language instructions and memory addresses. So the language is fundamentally engineered in a way that doesn't allow for what you want.

Some other languages, like Python, preserve the name information. They don't compile in the same way as C++. There's usually a lot more interpreting at runtime happening. At some fundamental level, the variables in a Python program probably end up in some kind of underlying data structure - perhaps a map of names to values. That way the language can support on-the-fly lookups and such of variable names.

So at best in C++, you can engineer your own data structures or framework in a similar manner - to hold data with an associated name and then be able to query by name to retrieve the relevant data. (Or of course, you could search for someone else who has already implemented such a thing.)

Answer 2

The CPython implementation - the implementation of Python written in C - uses the C language to implement a certain kind of variable lookup that makes Python behave like Python and not like, say, C or BASIC.

So, you can certainly use C or C++ to implement string-named variable lookup. Just don't expect the compiler to recognize it, because it's not part of the C/C++ language. The end product - the compiled executable - will support the concept, though.

For example, in C++, using Boost:

std::map<std::string, boost::variant<std::string, int>> _;

_["name"] = "user";
_["index"] = 1938107;

std::cout << _["name"] << _["index"] << std::endl;

Output:

user1938107

You can also make it interoperate with regular variables known to the compiler, if but there are no off-the-shelf classes to help you there.

Answer 3

A screwdriver can't drill a hole. But you can use a screwdriver to build a drill.

Answer 4

In another way, I think you CAN create some TYPE of variables from string or something using some trick, like in the Factory Design Pattern, but, that's not a language feature, you will need to register the type's name and the type's creator function first.

std::map<string, creating_function> lookup_table;
lookup_table["class_1"] = &class_1_creator;  // register type and creator

obj_instance& Create(string type_name, parameters_passed_to_the_creator) // create a variable
{
    return lookup_table[type_name](parameters_passed_to_the_creator);
}