I'm working with webview (found here: https://github.com/webview/webview) to create a c++ program with a html/js gui. In order to call c++ functions from js, they must be in the form std::string function(std::string). This is fairly trivial for free functions, however it seems not so trivial if you want to pass a pointer to a member function.
So, I've written a class which stores a reference to the object and its function, and in the constructor webview::bind is called with lambda function which parses the string input, and calls the function. It then converts the result to a string (I'm assuming this will be used for cases where there is a function that can do this) and returns that.
Now strangely this seems to work for a member function with no parameters (E.g. testcase::num below) but if I include one with parameters I get the error:
testcase.cpp:14:22: error: no matching function for call to 'invoke'
auto r = std::invoke(f, o, std::forward<Args>(get<Args>(args))...);
^~~~~~~~~~~
testcase.cpp:55:15: note: in instantiation of member function 'binder<testclass, int (testclass::*)(int, int)>::binder' requested here
auto b2 = binder(w, tc, &testclass::add, "add");
^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/functional:2845:1: note: candidate template ignored: substitution failure [with _Fn = int
(testclass::*&)(int, int), _Args = <testclass &>]: no type named 'type' in 'std::__1::invoke_result<int (testclass::*&)(int, int), testclass &>'
invoke(_Fn&& __f, _Args&&... __args)
^
Below the example code, which is compiled on a Mac with g++ testcase.cpp -o testcase -std=c++17 -framework WebKit:
#include <string>
#include <sstream>
#include <functional>
#include <iostream>
#include "webview.h"
template<class Obj, class F, class ...Args>
class binder{
public:
binder(webview::webview &w, Obj& obj, F func, std::string name) : _w(w), o(obj), f(func) {
_w.bind(name, [&](std::string s)->std::string {
std::stringstream args(s);
auto r = std::invoke(f, o, std::forward<Args>(get<Args>(args))...);
return std::to_string(r);
});
}
private:
template<class T>
static T get(std::istream& args){
T t; // must be default constructible
if(!(args >> t)){
args.clear();
throw std::invalid_argument("invalid argument to stream_function");
}
return t;
}
Obj& o;
F f;
webview::webview& _w;
};
class testclass {
public:
int add (int a, int b) {
return a+b;
}
int num () {
return 5;
}
};
int main() {
webview::webview w(true, nullptr);
w.set_title("test");
w.set_size(1200, 800, WEBVIEW_HINT_NONE);
testclass tc;
auto b1 = binder(w, tc, &testclass::num, "num"); // This compiles
auto b2 = binder(w, tc, &testclass::add, "add"); // This doesn't
w.navigate(R"(data:text/html,
<!doctype html>
<html>
<body>
<div id='num'></div>
<div id='add'></div>
</body>
<script>
window.onload = function() {
num('hello').then(function(res) {
document.getElementById('num').innerHTML = res;
console.log('num res', res);
});
add(1, 2).then(function(res) {
document.getElementById('add').innerHTML = res;
console.log('add res', res);
});
};
</script>
</html>
)");
w.run();
}
Thanks for your help
