Simple project

inc_foo.hpp

#include <string>

class Hpp {
	std::string s;
public:
	Hpp(std::string&& st):s(move(st)){}
	std::string str(){ return s; }
	size_t size();
	~Hpp(){}
};

meson.build

project('Foo', 'cpp')
inc = include_directories('inc')
subdir('src')

src_foo.cpp

#include <string>
#include <iostream>
#include "foo.hpp"

class Cpp {
	std::string s;
public:
	Cpp(std::string&& st):s(move(st)){}
	std::string str(){ return s; }
	size_t size();
	~Cpp(){}
};

size_t Cpp::size(){
	return s.size();
}

size_t Hpp::size(){
	return s.size();
}

int main(){
	Hpp h("foo");
	Cpp c("bar");
	std::cout<<"h="<<h.str()<<":"<<h.size()<<" c="<<c.str()<<":"<<c.size()<<std::endl;
	return 0;
}

src_meson.build

executable('foo', 'foo.cpp', include_directories: inc)