Wrap const char* in the Nim language

Nim language has a great FFI with C/C++ which makes it easy to use C/C++ libraries. This article introduces
how to wrap const char* in C/C++ backend.

In the Nim world, we often play with string which handles the dirty works for us such as resizing the buffer, creating/deleting the memory which can satisfy the most of our needs. However, if we want to interact with other languages, we need the power of cstring. Note that cstring doesn't mean "string in C", it means compatible string(char * in C/C++ backend, JS string in JavaScript backend and so on).

Nim doesn't provide const char * for us. But it is easy to make our own one.

You need to import const char* types from C/C++ backend using importc pragmas. Then make a distinct type for it.

type
  cstringConstImpl {.importc:"const char*".} = cstring
  constChar* = distinct cstringConstImpl

Now, let's look at a simple example using it.

First emit the c function, then write the function declaration(nodecl pragmas means that we
do not generate the function declaration in the output codes). Finally we construct a constChar object and
pass it to the fn proc.

{.emit: """
int fn(const char* a, char b) {
  return 1;
}
"""
.}

proc fn(a: constChar, b: char): int {.importc: "fn", nodecl.}

var x = constChar("abc")
doAssert fn(x, 'c') == 1
doAssert not compiles(fn("abc", 'b'))
doAssert not compiles(fn("abc".cstring, 'b'))

We could see that the fn proc doesn't accept string and cstring types and only the constChar type is allowed. After looking at the generated C codes using nim c -r --nimcache:nimcache app.nim or nim cpp -r --nimcache:nimcache app.nim, it does generate const char* x = "abc";.

Lastly let's look at an example with C++ backend, it has the same story as above.

Now we need to create a new .h file and overload the fn functions.

fun.h

int fn(const char* a, char b) {
  return 1;
}

int fn(char* a, char b) {
  return 2;
}

fun.nim

proc fn(a: constChar, b: char): int {.importcpp: "$1(@)", header: "fun.h".} = discard
proc fn(a: cstring, b: char): int {.importcpp: "$1(@)", header: "fun.h".} = discard


var a = constChar("abs")
doAssert fn(a, 'b') == 1
var b = cstring("abc")
doAssert fn(b, 'b') == 2

Run the codes above using nim cpp -r fun.nim and it works!

Reference:
https://github.com/nim-lang/Nim/issues/3720

Nim is a great language. Let's port and wrap more libraries for it.

https://github.com/nim-lang/needed-libraries/issues

https://github.com/nothings/single_file_libs