Re: Creating and using a Libray
Xavier Pegenaute <xpegenaute@gmail.com> writes:
Hi,
I am trying to create a library and distribute the library as a .so
file and a simplified header (without private data). I prepared a
basic example (see below).
The setter shows a segfault.
Maybe this is not the right way to do it? Any one knows what's wrong
here?
I am certainly not going to be able to give you an `expert' analysis
here, but I would say that this is very much *not* the way to do it. I
will add some comments below by way of isolating what is indeed wrong
with how you have coded your idea.
Thanks & Regards.
Xavi.
------------------ PRIVATE HEADER IN LIB ----------------------
#ifndef _TEST_H
#define _TEST_H
#include <vector>
using std::vector;
class Test {
private:
int testNumber;
vector<double> numbers;
public:
Test();
virtual ~Test();
const int get_testNumber() const;
void set_testNumber(int value);
void set_testNumber(vector<double> value);
};
#endif
Here, think about how objects of class Test are composed in terms of
their structure at run-time. What is the size of a Test instance? It
has certainly to reserve enough space for an integer data member, and
then there is the foot-print of the vector and its pointer to its own
representation. Certainly it has non-zero size.
------------------ PRIVATE CPP IN LIB ----------------------
#include "Test.h"
Test::Test() {
}
Test::~Test() {
}
const int Test::get_testNumber() const {
return testNumber;
}
void Test::set_testNumber(int value) {
testNumber = value;
}
void Test::set_testNumber(vector<double> value) {
numbers = value;
}
Here, the implementation in the library presents member functions that
interact with the data members of an instance, again knowing the size of
such, and its layout.
------------------- PUBLIC HEADER TO SEND WITH LIB -------------------
#ifndef _TEST_H
#define _TEST_H
#include <vector>
using std::vector;
class Test {
public:
Test();
virtual ~Test();
const int get_testNumber() const;
void set_testNumber(int value);
void set_testNumber(vector<double> value);
};
#endif
In your user code, however, you are providing a class that doesn't
*have* any data members. As far as any code that makes use of this
header is concerned there are none, and merely linking against the
library implementation of your *other* class (other, as in the real
sense of "not the same at all") does not magically provide them for the
objects created by your end user. Put simply again, what size do you
think an object of type Test created by your end user is? Has it
appropriately allocated space for the data members that your member
functions are soon to be accessing/writing to?
---------------------- Example code who uses the library
--------------------
#include "Test.h" // The public header
#include <iostream>
#include <vector>
#include <string>
using namespace std;
int main(){
vector<double> v(200, 2);
Test * t = new Test();
t->set_testNumber(3);
cout<<t->get_testNumber()<<endl;
t->set_testNumber(v); // <----------- Segfault in runtime!!!!
return 0;
}
The problem with the `trick' that you have attempted here - of providing
subtely, yet significantly, different class definitions to the library
code and then to the user - is that it will fool the compiler into not
suspecting that anything is wrong. The interface of your user
definition matches that of the library implementation and so everything
will link. (I certainly don't know of any compiler that will be able to
diagnose this.)
However, when you call your methods, you are invoking code written for an
instance of a class that has space correctly set aside for two data
members on an instance of a class that has no such members. With your
call to t->set_testNumber(v) you are calling code that is attempting to
write to a data member that t doesn't have.
To think about a correct way of achieving what you want - which, I am
supposing, is merely to hide the implementation details of your library
class from its users - take a look at something like the PIMPL idiom.
Regards
Paul Bibbings
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