Using macros to parse string to expression C++

Question

I tried to write a program that takes input as a string and then passes that string to a macro which is supposed to insert the string as a plain-text expression but the macro is not behaving as I would suspect.

#include <iostream>
#include <cmath>
#include <string>

#define  PARSE(a) a;


using namespace std;

int main() {
    string c ;
    int b;
    cin >> c;

    b = PARSE(c);

    cout << b;

    return 0;
}

This code will not compile, it gives an error saying that I cannot convert string to int, however PARSE(c) should not be treated like a string it should just be replaced by plain text.

The preprocessor runs even before the regular compiler does (hence the name preprocessor). It can't use runtime values. What you want is a plain old function to convert a string to int, which has plenty of existing presence on this site. — chris, Nov 17 '19 at 22:33
I suppose `#define PARSE(a) atoi(a);` (with a `#include ` at the top) would be considered cheating? — Jeremy Friesner, Nov 17 '19 at 22:36
Ok, I see. The preprocessor would just replace PARSE(c) with the plain-text c. — Ender_The_Xenocide, Nov 17 '19 at 22:38
What would be the easiest way to parse string to expression? — Ender_The_Xenocide, Nov 17 '19 at 22:39
Are you talking about a Python `eval`/`exec`/`compile` equivalent in C++? — Andreas is moving to Codidact, Nov 17 '19 at 23:39
One rule in C++ is that one should **avoid macros** as much as possible... — Phil1970, Nov 18 '19 at 00:34

Matt · Accepted Answer · 2019-11-17T23:06:21.543

1

The error means you are really trying to convert std::string (c) to int and it can't be done (b). If you want to access the plain text, once you are using std::string, you should call the c_str() method, example:

cout << c.c_str();

@edit: If you are trying to do something like "eval()" from PHP, you can check this: https://stackoverflow.com/a/11078610/12385171

edited Nov 17 '19 at 23:06

answered Nov 17 '19 at 22:38

Matt

53
4

I am not talking about plain-text in that sense, I wanted the pre-compiler to directly change the text in the uncompiled c++ file. – Ender_The_Xenocide Nov 17 '19 at 22:53
@Ender_The_Xenocide, I added a reference, tell me if it is what you want. – Matt Nov 17 '19 at 23:06

Using macros to parse string to expression C++

1 Answers1