How do I fix this c++ expected expression error? - c++

Question

I am making something in c++, it doesn't have any errors visible in Visual Studio code, but when I use g++ to be able to execute it, I get this error:
In file included from Main.cpp:6: In file included from ./Filechange/Filechange.hpp:1: ./Filechange/Filechange.cpp:14:24: error: expected expression
std::thread first ([&wtime,&f,&fn]() mutable {
^ Main.cpp:16:33: error: expected expression
OnFilechange("FileEvent", 0.5, [](char* txt){
^ 2 errors generated.
These are the files:
Main.cpp:
#include <lua.hpp>
#include <iostream>
#include <chrono>
#include <thread>
#include <stdio.h>
#include "Filechange/Filechange.hpp"
void wait(int seconds)
{
std::this_thread::sleep_for(std::chrono::seconds(seconds));
}
int main(int argc, char const *argv[])
{
lua_State *State = luaL_newstate();
OnFilechange("FileEvent", 0.5, [](char* txt){
std::cout << txt << std::endl;
});
lua_close(State);
return 0;
}
Filechange.cpp:
#include <thread>
#include <stdio.h>
#include <stdlib.h>
#include <chrono>
#include <string>
#include <fstream>
char* StringToChar(std::string str){
char* Array = new char[str.length() + 1];
strcpy(Array,str.c_str());
return Array;
}
void OnFilechange(const char *f, float wtime, void (*fn)(char* txt)){
std::thread first ([&wtime,&f,&fn]() mutable {
std::ifstream file(f);
std::string str;
std::string filecontents;
while (std::getline(file,str)){
filecontents += str;
filecontents.push_back('\n');
}
char* LastContents = StringToChar(filecontents);
char* CurrentContents = StringToChar(filecontents);
while (true){
if (wtime != 0){
std::this_thread::sleep_for(std::chrono::milliseconds(int(wtime*1000)));
}
filecontents = "";
while (std::getline(file,str)){
filecontents += str;
filecontents.push_back('\n');
}
CurrentContents = StringToChar(filecontents);
if (strcmp(LastContents, CurrentContents) != 0){
LastContents = StringToChar(filecontents);
fn(StringToChar(filecontents));
}
}
});
}
Filechange.hpp:
#include "Filechange.cpp"
#ifndef FILECHANGE_HPP
#define FILECHANGE_HPP
#include <thread>
#include <stdio.h>
#include <stdlib.h>
#include <chrono>
#include <string>
#include <fstream>
void OnFilechange(const char *f,float wtime,void (*fn)(char txt));
#endif
There's also a extension less file named FileEvent which will change in the runtime using other code files.
The Filechange.cpp and Filechange.hpp are in a folder named "Filechange"

Answer 1

This function:
void OnFilechange(const char *f, float wtime, void (*fn)(char* txt))
expects a function pointer, and a lambda in g++ is not implemented as a function pointer. Instead, you should declare the function to take a std::function, as in:
void OnFilechange(const char *f, float wtime, std::function<void(char *)> fn)
You may also need #include <functional> to get the declaration of std::function.

Answer 2

use -std=c++17 in g++ if possible as g++ defaulted to c++98