Making a function call to a function whose name is inside a string - C++

Question

I am extracting a string from a .txt file and saving it in a variable:

std::string line = "The king's name is getKingName()";

Lets assume that getKingName() is a function that returns a King class' name data member.

How can I make a call to getKingName() when the string variable looks like that?

Does this answer your question? Is there C/C++ equivalent of eval("function(arg1, arg2)")? — GoodDeeds
– GoodDeeds, Commented Jan 24, 2020 at 12:29
I have seen those answers before and I couldn't really figure out if I could apply them to my case. Could you provide example code using the example string above? — Pete
– Pete, Commented Jan 24, 2020 at 12:36
You need some way to parse the function name out of the string (depends on your input formatting). Then, you can create a map that maps the function name string to the function, and call the map with the parsed function name. — GoodDeeds
– GoodDeeds, Commented Jan 24, 2020 at 12:38
First, which is the problem? Finding the name of the method or invoking the method once you already found it? And what have you tried doing/what is the specific problem you found? — SJuan76
– SJuan76, Commented Jan 24, 2020 at 12:44

Shubham Patel · Accepted Answer · 2020-01-24 14:46:18Z

As far as I know, C++ does not provide such kind of functionality to interpolate functions call inside a string. All you can do implement your own logic to do that. Like,

1) define all the valid methods like this,

string getKingName(){ return "Some name"; } string otherMethods(){ return "other values"; }

2) One helper method for mapping of function call

string whomToCall(string methodName){ switch(methodName){ case "getKingName()": return getKingName(); break; case "otherMethods()": return otherMethods(); break; default: return "No such method exist"; } }

3) break the line in tokens(words), read one by one and check for following condition also if token starts with any alphabetical character and ends with "()" substring

 istringstream ss(line); do { string token; ss >> token; if(isMethod(token)) cout << whomToCall(token) << " "; else cout << token<< " "; } while (ss);

4) isMethod() to check if token's value can be a valid method name

bool isMethod(string token){ int n= token.length(); return isalpha(token[0]) && token[n-2]=='(' && token[n-1] == ')' ; }

RoQuOTriX · Accepted Answer · 2020-01-24 12:40:00Z

1

This would be the easiest solution, but I think your problem consists of several such calls?

std::string line = "The king's name is getKingName()"; if (line.find("getKingName()") != std::string::npos) { King name = getKingName(); }

edited Jan 24, 2020 at 12:40

answered Jan 24, 2020 at 12:32

RoQuOTriX

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6 Comments

Pete Over a year ago

The problem is that the function call could be pretty much anywhere in the string - beginning/middle/end of string.

RoQuOTriX Over a year ago

Then use the find() function. I will edit my code

Marc Stroebel Over a year ago

or use a regex like [A-Za-z]+\(\)

RoQuOTriX Over a year ago

@MarcStröbel I don't like the usage of regular expressions, for such an "easy" task. Regular expressions often lead to unwanted "features"

Tiger4Hire Over a year ago

Also a better "modern" style would be to use std::map<std::string, std::function>. Use this implace of a swtch statement. Consider std::hash_map for speed. Std::function lets you use lambda functions, which can be convenient.

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Tiger4Hire · Accepted Answer · 2020-01-24 12:51:23Z

Amended

This answer is a little off subject. I will leave it up, because others might find it relevant, but I agree with other answers, a simple map->function will work better for your case.

This is not supported by C++. C++ is not an interpreted language. If you you want to do things like this, why not use an interpreted language, which do these sorts of things by default. Languages like lua are designed to call C/C++ functions with an interpreted language, with a small overhead.

However, if you really need to do this, it is possible, depending on your operating system. For example,

On windows start with dbghelp. You will need to build a pdb, (e.g. build with symbols).
On linux, you will also need to build symbols (-g), and use something like dlsym see here for a discussion.

That said, there are lots of gotchas doing it this way. Optimization can get in the way (best to disable them). Also best to avoid dynamic linking (prefer static). You will also need to cope with C++ name mangling (the name of the function is not the name of your function in C++). see https://blog.oakbits.com/how-to-mangle-and-demangle-a-c-method-name.html.

I support your answer, but I think OP has a fixed size of possibilities. He does not need dynamically call every function hin his program
I agree, on reflection this answer is too complex. I'm wondering how to edit it to make this clearer.

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Making a function call to a function whose name is inside a string - C++

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Amended

This answer is a little off subject. I will leave it up, because others might find it relevant, but I agree with other answers, a simple map->function will work better for your case.

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Amended

This answer is a little off subject. I will leave it up, because others might find it relevant, but I agree with other answers, a simple map->function will work better for your case.

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