How to write a simple interpreter in C++?

Question

How to write a simple interpreter for a simple language in C++ that gets a .txt file as input and runs that.

** Just for + - / * out functions **

Input (Source code) :

int a,b,c;
a=2;
b=4;
c=a+b;
out c;

Interpreter output :

How i can do this ? Any help or source code appreciated.

Sorry if u didn't get my question, My English is so bad. Just ignore the " Interpreter " for a sec . I want to write an console program in C++ that gets a txt file as source code and recognize the basic math and "out" command . is it possible in C++ or not ?

There is not too many possible answers: there are specific math to solve this questions and the way to implement that is not wider than for any other question. How to remove this "Hold"? — Adrian Maire, Oct 09 '15 at 07:16
If you don't want to reinvent the wheel: http://boost-spirit.com/ — hyde, Oct 09 '15 at 07:16
Look into embeddable interpreters like [Lua](http://lua.org/) or [Guile](http://www.gnu.org/software/guile/). See also [this](http://stackoverflow.com/a/27554300/841108) & [that](http://stackoverflow.com/a/25317439/841108) & [that](http://stackoverflow.com/a/29816531/841108) and the references I'm giving there. — Basile Starynkevitch, Oct 09 '15 at 07:17
@AdrianMaire The question is basically "how do I create interpreter in C++ for a language like this?", and I'd say that is definitely in the "too broad" category. There are just too many valid ways to do it, especially with language like C++. — hyde, Oct 09 '15 at 07:22
@Emperor280 your question is off-topic here because it is not related to source code. And it would be off-topic on [Programmers](http://programmers.stackexchange.com/) because it is too broad and does not show any prior research — Basile Starynkevitch, Oct 09 '15 at 07:22
@hyde: No, look my answer, the path is quite defined. You could also define which of the syntax to use, but I did not get so far. Supposing for example a LALR(1) you could even set a pseudo-algorithm. — Adrian Maire, Oct 09 '15 at 07:24
@AdrianMaire: the bulk of any interpreter is not [parsing](https://en.wikipedia.org/wiki/Parsing), but interpreting, I.e. defining the [semantics](https://en.wikipedia.org/wiki/Semantics_%28computer_science%29) of the scripting language, then coding the interpreter. — Basile Starynkevitch, Oct 09 '15 at 07:25
@AdrianMaire The question asks about C++, you answer doesn't really cover that. But perhaps it's the question, which should be edited to use [tag:pseudocode], or just not have "language" tag in it. — hyde, Oct 09 '15 at 07:27
@hyde: probably you are right and the C++ tag should be removed or switched to pseudo-code. I think we should be more careful in tagging new users, that is very de-motivating. Usually an explanation/correction is more constructive. — Adrian Maire, Oct 09 '15 at 07:37
*"is it possible in C++ or not ?"*, obviously it is possible... In about 100 different ways. If you want to do it yourself from scratch, work in small steps (as always in programming). You *could* start by writing a tokenizer, which recognizes your two keywords (`int` and `out`), numbers and operators (`=+-*/`). You could create a token class, and then have `std::vector`, where you put the tokens. Once you have code which can create such a vector (and give syntax/parse error message when parsing fails), you can start looking into what to do with the tokens... — hyde, Oct 09 '15 at 09:46

Adrian Maire · Answer 1 · 2015-10-09T07:08:06.833

An interpreter is usually made in 3 steps:

Lexical Recognize specific simple shemes, like keyword, numbers, symbols, etc. The standard way is to define a regular expression for each lexer. There are well defined algorithm there outside to transform a reg-exp in a state machine so you may recognize any work in your input.

This is made by:

Create a state machine from each regexp
Join all state machines
Make the state machine deterministic
Make the state machine minimal.

Syntax In this part, you get the sequence of lexer as input and create a tree with them. Depending on the complexity of your language, there are different types: top->down or down->top. (speaking about LR, LL, LALR, etc.)

file
|...
|- c=a+b
|  |- a+b
      |- a
      |- +
      |- b
   |- =
   |-c

Semantic Go trough your tree and make the operations, so for example in the a+b, you get the a, the b and you sum them, then you return to the above node, and set the value to c.

Final note: Be careful to design a powerful error/warning mechanism from the starting: type of error, full description, line and char where the mistake is detected, level of the error/warning, etc.

Also, could be interesting to provide for each node the parsed input (string), the interpreted content (LEX_NUMBER) and the interpreted value (4).

That was helpful , Thanks. – Emperor280 Oct 09 '15 at 07:09 — Emperor280, Oct 09 '15 at 07:09

How to write a simple interpreter in C++?

1 Answers1