I am trying to use the PtInRect function in Win32, which takes a POINT object. I have x and y coordinates, but I don't know how I create a POINT from those. I tried looking it up, but could only find this documentation, which does not contain the information I want.
Thank you for your help.
Problem:
You're creating trouble for yourself because of attempting to dynamically allocate the POINT object with malloc. BTW you do not always need dynamic allocation and to initialize any struct you do not need to use malloc as you pointed in the comments, you can just create the object directly.
Solution:
As tkausl pointed in the comments, POINT is created in the same way as any other struct.
Some examples on how creating a POINT:
int main()
{
int someX = 100;
int someY = 100;
RECT testRect1 = {0,0,200,200};
RECT testRect2 = {150,150,350,350};
POINT p1{someX,someY};
POINT p2 = {someX,someY};
POINT p3;
p3.x = someX;
p3.y = someY;
std::cout << (bool) PtInRect(&testRect1,p1) << std::endl;
std::cout << (bool) PtInRect(&testRect1,p2) << std::endl;
std::cout << (bool) PtInRect(&testRect1,p3) << std::endl;
std::cout << (bool) PtInRect(&testRect1,{someX,someY}) << std::endl;
std::cout << (bool) PtInRect(&testRect2,p1) << std::endl;
std::cout << (bool) PtInRect(&testRect2,p2) << std::endl;
std::cout << (bool) PtInRect(&testRect2,p3) << std::endl;
std::cout << (bool) PtInRect(&testRect2,{someX,someY}) << std::endl;
}
Related
I need to pass structure to the function Constraint::AddFixedOrientationAxis, however when I check the passed data their values are completely wrong. I have tried to use different datatypes but without any luck.
typedef struct{
size_t idx;
size_t axis_idx;
double axis_vector_1;
double axis_vector_2;
double axis_vector_3;
}AddFixedOrientationAxisData;
double Constraint::AddFixedOrientationAxis(const std::vector<double> &x, std::vector<double> &grad, void *data)
{
Eigen::VectorXd fixed_rot(3);
AddFixedOrientationAxisData *d = reinterpret_cast<AddFixedOrientationAxisData*>(data);
auto idx = d->idx;
auto axis_idx = d->axis_idx; // 0->x, 1->y, 2->z
fixed_rot << d->axis_vector_1, d->axis_vector_2, d->axis_vector_3;
std::cout << "idx: " << idx << std::endl;
std::cout << "axis: " << axis_idx << std::endl;
std::cout << "fixed_rot: " << fixed_rot << std::endl;
}
In the main, I call use it the same way as the tutorial is:
AddFixedOrientationAxisData fixed_orient_constraint_data;
fixed_orient_constraint_data.idx = 0;
fixed_orient_constraint_data.axis_idx = 0;
fixed_orient_constraint_data.axis_vector_1 = FK_q(0,0);
fixed_orient_constraint_data.axis_vector_2 = FK_q(1,0);
fixed_orient_constraint_data.axis_vector_3 = FK_q(2,0);
opt.add_equality_constraint(Constraint::AddFixedOrientationAxis, &fixed_orient_constraint_data);
The terminal output is:
idx: 93901286131024
axis: 93901286131080
fixed_rot:
4.63934e-310
-0.54938 //interestingly, this is the correct value
0.00838157 //interestingly, this is the correct value
As #{Some programmer dude} told me in the comments, the problem was that the variable was not alive when the function was called.
I have two classes, let's call them A and B
class A:
{
public:
//Some functions
A *getNewA() const;
private:
//some attributes
}
class B:
{
public:
//Some functions
private:
A &reftoA;
}
In the main code, I must generate a new A thanks to the A::getNewA() method. And this must go to B::reftoA, as written in class B.
Here is the A::getNewA() method :
A *A::getNewA()
{
A *newA = new A;
return newA;
}
OK. So now I call getNewA and want to store the results in reftoA, which is a reference to A. In a B function (which take a reference to A as parameter)
B::foo(A ¶mA)
{
reftoA = *(paramA.getNewA());
}
I thought this should have been working, but it won't.
Because when dereferencing, reftoA will always take the this object and not the new allocated object.
Let's be clearer and let's modify the functions to output the results
A * A::getNewA()
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return A;
}
void B::foo(A ¶mA)
{
reftoA = *(paramA.getNewA());
std::cout << "new generated pointer " << &reftoA << std::endl;
}
Here is one of the output :
New pointer : 004FFAEC
this pointer: 0069D888
New generated pointer : 0069D888 //Expected : 004FFAEC
I can't get this "new generated pointer" to be the same than the new pointer the A::getNewA() returns after having allocated the memory. Of course, I guess there is some point with dereferencing the pointer to store it in a reference.
I know reference are used with existing object. Maybe the new object A::getNewA() should allocate memory for won't work as I expected.
I could use pointer instead reference in B::foo(), I know, but I can't
I think I am misunderstanding something about refrence and pointer, but I don't know what.
Any help greatly appreciated
The problem is that you can not reassign a reference. You can change only the value of the referenced object.
So you have to initialize the reference in the initializer list of the constructor of the class B.
Take into account that there is a typo in your code snippet
A*A::getNewA()
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return A;
^^^^^^^^^
}
I think you mean
A*A::getNewA() const
^^^^^
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return newA;
^^^^^^^^^^^
}
Always try to provide a verifiable complete example.
Here is a demonstrative program
#include <iostream>
class A
{
public :
//Some functions
A* getNewA() const
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return newA;
}
private :
//some attributes
};
class B
{
public :
B( const A& a ) : reftoA( *a.getNewA() )
{
std::cout << "&reftoA " << &reftoA << std::endl;
}
private :
A& reftoA;
};
int main()
{
A a;
B b( a );
return 0;
}
Its output is
New pointer 0x2b392afbec20
this pointer0x7ffd287ad0af
&reftoA 0x2b392afbec20
As you can see the values of the New pointer and &reftoA are equal each other.
To make it more clear consider a very simple example
#include <iostream>
int main()
{
int x = 10;
int y = 20;
int &r = x;
r = y;
std::cout << "x = " << x << std::endl;
std::cout << "y = " << y << std::endl;
std::cout << "r = " << r << std::endl;
std::cout << std::endl;
std::cout << "&x = " << &x << std::endl;
std::cout << "&y = " << &y << std::endl;
std::cout << "&r = " << &r << std::endl;
return 0;
}
The program output is
x = 20
y = 20
r = 20
&x = 0x7ffd88ad47a8
&y = 0x7ffd88ad47ac
&r = 0x7ffd88ad47a8
This statement
r = y;
did not force the reference to refer the object y. It just reassigned the value of the referenced object x.
References have to be initialized when they are created.
Yes, you are misunderstanding something.
getNewA() is returning a pointer. it's not a smart pointer, you want to look into those and that's all I'll say on the matter.
on returning a pointer, you must keep a reference to this pointer else you will be unable to delete it and you'll get a memory leak. Thus you MUST have somewhere A* a = A::getNewA() and then later, when you no longer need it delete a;
Where you need to pass a reference to A, you can do foo(*a) which will dereference the pointer and pass a reference to the object it's pointing to.
But in summary, for all new code, smart pointers; there's no excuse to not use them.
Side note: Your code example had a few other issues; such as getNewA wasn't static; I'm going to take the code as a working example of your understanding, and not a working example.
Edit: On re-reading your example, the getNewA is intentionally non-static. I think this question is actually an XY problem (ie you're asking a question you've forced yourself into but isn't your actual problem); but I hope this addresses your misunderstanding of pointers and references.
You are not returning the pointer in the getNewA-Method
A* A::getNewA()
{
A *newA = new A;
return A; // you are returning A and not newA
}
And if you want to reassign the reference to a you can use a std::reference_wrapper
class B :
{
public :
void foo(A& paramA) {
reftoA = *(paramA.getNewA());
}
private :
std::reference_wrapper<A> reftoA;
}
This question already has answers here:
What are the differences between a pointer variable and a reference variable?
(44 answers)
Closed 9 years ago.
I have objects that I put into a std::vector. Later on I need to iterate through the vector and change some member variables in the objects in each position.
I think I want to pass the object once I have it by reference to a function to operate on it, but I seem to be getting an error:
Non-const lvalue reference to type 'Object' cannot bind to a value of unrelated type 'Object *'
Here is the general gist with code between omitted:
Object* o1 = Object::createWithLocation(p.x, p.y);
v.push_back(o1);
// later on
for (int f=0; f < v.size(); f++)
{
Object* obj1 = v.at(f);
addChild(h->createLayer(obj1), 3); // add the HUD
}
createLayer is defined at:
static PlantingHUD* createLayer(Object &o);
Can anyone explain my confusion between pointers and passing by reference? Do I have to do a cast of some sort?
static PlantingHUD* createLayer(Object &o);
this method need a reference to Object as the parameter,
but your input is a pointer.
Object* obj1 = v.at(f);
addChild(h->createLayer(obj1), 3); // add the HUD
That's the problem.
void foo(Object o)
Declares a function, foo, which will begin execution with a fresh, new, instance of class 'Object' called 'o'.
This is called "passing by value", but it's more accurately 'copying' because what foo receives is it's own, personal copy of the Object instances we call foo with. When "foo" ends, the "Object o" it knew, fed and put through school, will cease to be.
void foo(Object& o)
Declares a function, foo, which will begin executing with a reference to an existing instance of an 'Object', this reference will be called 'o'. If you poke or prod it, you will be changing the original.
This is called "pass by reference".
void foo(Object* o)
Declares a function, foo, which will begin executing with a variable, called "o", containing the address of what is supposed to be an instance of "Object". If you change this variable, by doing something like "o = nullptr", it will only affect the way things look inside foo. But if you send Samuel L Jackson to the address, he can deliver furious vengance that lasts beyond the lifetime of foo.
void foo(Object*& o)
Declares a function, foo, which will begin executing with a variable called "o", which is a reference to a pointer to an instance of object o - it's like an alias, except that without compiler optimization, it's actually implemented by the compiler using a sort of pointer.
Lets try these separately.
#include <iostream>
#include <cstdint>
struct Object
{
int m_i;
void event(const char* what, const char* where)
{
std::cout <<
what<< " " << (void*)this <<
" value " << m_i <<
" via " << where <<
std::endl;
}
// Construct an object with a specific value.
Object(int i) : m_i(i)
{
event("Constructed", "Operator(int i)");
}
// This is called the copy constructor, create one object from another.
Object(const Object& rhs) : m_i(rhs.m_i)
{
event("Constructed", "Operator(const Object&)");
}
// This is how to handle Object o1, o2; o1 = o2;
Object& operator=(const Object& rhs)
{
m_i = rhs.m_i;
event("Assigned", "operator=");
return *this;
}
// Handle destruction of an instance.
~Object() { event("Destructed", "~Object"); }
};
void foo1(Object o)
{
std::cout << "Entered foo1, my o has value " << o.m_i << std::endl;
// poke our local o
o.m_i += 42;
std::cout << "I changed o.m_i, it is " << o.m_i << std::endl;
}
void foo2(Object* o)
{
std::cout << "Foo2 starts with a pointer, it's value is " << (uintptr_t)o << std::endl;
std::cout << "That's an address: " << (void*)o << std::endl;
std::cout << "m_i of o has the value " << o->m_i << std::endl;
o->m_i += 42;
std::cout << "I've changed it tho, now it's " << o->m_i << std::endl;
}
void foo3(Object& o)
{
std::cout << "foo3 begins with a reference called o, " << std::endl <<
"which is sort of like a pointer but the compiler does some magic " << std::endl <<
"and we can use it like a local concrete object. " <<
std::endl <<
"Right now o.m_i is " << o.m_i <<
std::endl;
o.m_i += 42;
std::cout << "Only now, it is " << o.m_i << std::endl;
}
void foo4(Object*& o)
{
std::cout << "foo4 begins with a reference to a pointer, " << std::endl <<
"the pointer has the value " << (uintptr_t)o << " which is " <<
(void*)o <<
std::endl <<
"But the pointer points to an Object with m_i of " << o->m_i << std::endl <<
"which we accessed with '->' because the reference is to a pointer, " <<
"not to an Object." <<
std::endl;
o->m_i += 42;
std::cout << "I poked o's m_i and now it is " << o->m_i << std::endl;
// Now for something really dastardly.
o = new Object(999);
std::cout << "I just changed the local o to point to a new object, " <<
(uintptr_t)o << " or " << (void*)o << " with m_i " << o->m_i <<
std::endl;
}
int main()
{
std::cout << "Creating our first objects." << std::endl;
Object o1(100), o2(200);
std::cout << "Calling foo1 with o1" << std::endl;
foo1(o1);
std::cout << "back in main, o1.m_i is " << o1.m_i << std::endl;
std::cout << "Calling foo2 with &o1" << std::endl;
foo2(&o1);
std::cout << "back in main, o1.m_i is " << o1.m_i << std::endl;
std::cout << "Calling foo3(o2), which looks like the way we called foo1." << std::endl;
foo3(o2);
std::cout << "back in main, o2.m_i is " << o2.m_i << std::endl;
std::cout << "Creating our pointer." << std::endl;
Object* optr;
std::cout << "Setting it to point to 'o2'" << std::endl;
optr = &o2;
std::cout << "optr now has the value " << (uintptr_t)optr <<
" which is the address " << (void*)optr <<
" which points to an Object with m_i = " << optr->m_i <<
std::endl;
foo4(optr);
std::cout << "back in main, o2 has the value " << o2.m_i << std::endl <<
"and now optr has the value " << (uintptr_t)optr << std::endl <<
"and optr->m_i is now " << optr->m_i <<
std::endl;
if (optr != &o2)
delete optr; // otherwise we'd technically be leaking memory.
return 0;
}
Live demo on ideone.com.
Passing by Value
This term confuses people early in their C++ development because, in lay terms, it sounds like this is what "Object& foo" would do.
The term "pass by value" actually arises from what the language has to do to call such a function, to value-wise copy the whole of the original object/struct onto the stack or, in the case where a copy ctor is available, forward them to a value-wise constructor and recreate a copy of the original, value-by-value.
Pass-by-value should be used for most simple cases where you do not want side-effects on the values in your current scope from the function you are calling.
bool checkWidthdrawl(Dollars balance, Dollars amountToWithdraw)
{
// it's safe for me to change "balance" here because balance is mine
}
vs
bool checkWidthdrawl(Dollars& balance, Dollars amountToWithdraw)
{
balance -= amountToWithdraw;
if (balance < 0)
std::complaint << "My account seems to be missing $" << amountToWithdraw;
}
However, passing by reference can become expensive.
struct FourK { char a[1024], b[1024], c[1024], d[1024]; }
If you pass this around by value all day, you risk blowing up your stack at some point, as well as spending daft amounts of time copying all those bytes.
void foo(int i); // Unless you need to see the changes to i, this is perfectly fine.
void foo(FourK f); // Someone should hunt you down and yell "PEANUT" in your ear.
Passing by reference
References are really a contract over the pointer system that allow the language to ensure you're really talking about a concrete instance of an object, and thus allow you to refer to a pre-existing instance of a value outside of a function.
Of course, there are ways to break this, but the language tries very, very hard to make them difficult to do. For example, try adding this to the above code:
Object& makeObjectNotWar(int i)
{
Object thisObjectGoesAway(i);
return thisObjectGoesAway /*right about now*/;
}
You can also provide callers with an assurance that the function won't have any side effects on a variable with the "const" modifier.
void fooc(const Object& o)
{
o.m_i += 42; // Error
}
You can even use that within a function as a hint to yourself (and the compiler) that you don't want to accidentally change a value, here's a case where it can provide an optimization hint to the compiler:
std::vector<int> foo;
add1000valuesTo(foo);
const size_t fooSize = foo.size();
for (size_t i = 0; i < fooSize; ++i) {
// ... stuff you're sure won't decrease foo.size()
}
Without the const fooSize
for (size_t i = 0; i < foo.size(); ++i) {
The compiler has to start by assuming that "foo.size()" could be changed at any given iteration of the loop. It can probably figure out that it doesn't, but by giving it the hint, you've saved a little compile time, possibly improved your performance, and made it easier for a human to tell exactly what behavior you expected. Downside: If your loop does actually change the size of foo, you'll find out by bug reports :(
One last thing to know about pass-by-reference is that C++ references aren't protected or "ref counted". The language only promises that a reference will be valid for the duration of its scope, so long as you don't do anything stupid like, say, call something that deletes the object.
// Author intended this function to be called
// by the owner of a Dog.
void doneWithFoo(Dog& dog)
{
Dog* deadDog = &dog;
delete deadDog;
}
Rover& Babysitter::babysitDog(Dog& rover, int hours)
{
rover.feed(FeedType::Donut);
if (rover.pooped())
doneWithDog(rover);
// ...
return rover; // I have a bad feeling about this.
}
Obviously, you're not expecting "babysitDog" to result in the dog being disposed of. But bear in mind that because we passed in a reference, it to "babysitDog" that it's also gone from the caller too, and if that was using a reference... rover's dead, Dave, dead.
As with pointers, if you're going to store references beyond the scope in which you have access to them, then you become responsible for making sure the objects being referenced stick around or that the references are removed from the container before the objects do go away.
Am i doing this right, I want a map with a Integer as key, and struct as value. What is the easiest way to, say I want the object at 1. How do I retrieve the value of isIncluded? The last two lines in the code, I tried doing it, but then I realized I don´t really know what is the way to retrieving values of structs in a numbered Map array.
Do I need to call cells.get(1) and assign that to a new temporarely struct to get its values?
/** set ups cells map. with initial state of all cells and their info*/
void setExcludedCells (int dimension)
{
// Sets initial state for cells
cellInfo getCellInfo;
getCellInfo.isIncluded = false;
getCellInfo.north = 0;
getCellInfo.south = 0;
getCellInfo.west = 0;
getCellInfo.east = 0;
for (int i = 1; i <= (pow(dimension, 2)); i++)
{
cells.put(i, getCellInfo);
}
cout << "Cells map initialized. Set [" << + cells.size() << "] cells to excluded: " << endl;
cells.get(getCellInfo.isIncluded);
cells.get(1);
}
the Map, is declared as an private instance variable like this:
struct cellInfo {
bool isIncluded;
int north; // If value is 0, that direction is not applicable (border of grid).
int south;
int west;
int east;
};
Map<int, cellInfo> cells; // Keeps track over included /excluded cells
From the documentation for Map, it appears that .get() returns a ValueType.
You would use it thus:
// Display item #1
std::cout << cells.get(1).isIncluded << "\n";
std::cout << cells.get(1).north << "\n";
Or, since the lookup is relatively expensive, you could copy it to a local variable:
// Display item #1 via initialized local variable
cellInfo ci = cells.get(1);
std::cout << ci.isIncluded << " " << ci.north << "\n";
// Display item #2 via assigned-to local variable
ci = cells.get(2);
std::cout << ci.isIncluded << " " << ci.north << "\n";
My best advice is to use the standard library's std::map data structure instead:
// Expensive way with multiple lookups:
std::cout << cells[1].isIncluded << " " << cells[1].north << "\n";
// Cheap way with one lookup and no copies
const cellinfo& ci(maps[1]);
std::cout << ci.isIncluded << " " << ci.north << "\n";
I define this structure:
struct s_molecule
{
std::string res_name;
std::vector<t_particle> my_particles;
std::vector<t_bond> my_bonds;
std::vector<t_angle> my_angles;
std::vector<t_dihedral> my_dihedrals;
s_molecule& operator=(const s_molecule &to_assign)
{
res_name = to_assign.res_name;
my_particles = to_assign.my_particles;
my_bonds = to_assign.my_bonds;
my_angles = to_assign.my_angles;
my_dihedrals = to_assign.my_dihedrals;
return *this;
}
};
and these structures:
typedef struct s_particle
{
t_coordinates position;
double charge;
double mass;
std::string name;
std::vector<t_lj_param>::iterator my_particle_kind_iter;
s_particle& operator=(const s_particle &to_assign)
{
position = to_assign.position;
charge = to_assign.charge;
mass = to_assign.mass;
name = to_assign.name;
my_particle_kind_iter = to_assign.my_particle_kind_iter;
return *this;
}
} t_particle;
struct s_bond
{
t_particle * particle_1;
t_particle * particle_2;
std::vector<t_bond_param>::iterator my_bond_kind_iter;
s_bond& operator=(const s_bond &to_assign)
{
particle_1 = to_assign.particle_1;
particle_2 = to_assign.particle_2;
my_bond_kind_iter = to_assign.my_bond_kind_iter;
return *this;
}
};
and then in my code I return a pointer to an s_molecule (typedef'd to t_molecule, but still).
Using this pointer I can get this code to work:
for (unsigned int i = 0;
i < current_molecule->my_particles.size();
i++)
{
std::cout << "Particle "
<< current_molecule->my_particles[i].name << std::endl
<< "Charge: "
<< current_molecule->my_particles[i].charge << std::endl
<< "Mass: "
<< current_molecule->my_particles[i].mass << std::endl
<< "Particle Kind Name: "
<< (*current_molecule->my_particles[i].my_particle_kind_iter).atom_kind_name
<< std::endl
<< "x: " << current_molecule->my_particles[i].position.x
<< " y: " << current_molecule->my_particles[i].position.y
#ifdef USE_3D_GEOM
<< "z: " << current_molecule->my_particles[i].position.z
#endif
<< std::endl;
}
If I replace it with:
for (std::vector<t_particle>::iterator it = current_molecule->my_particles.begin();
it !=current_molecule->my_particles.end();
it++)
{
std::cout << "Particle "
<< (*it).name << std::endl
<< "Charge: "
<< (*it).charge << std::endl
<< "Mass: "
<< (*it).mass << std::endl
<< "Particle Kind Name: "
<< (*(*it).my_particle_kind_iter).atom_kind_name
<< std::endl
<< "x: " << (*it).position.x
<< " y: " << (*it).position.y
#ifdef USE_3D_GEOM
<< "z: " << (*it).position.z
#endif
<< std::endl;
}
I now get nasty segfaults...
Not to put too much here, but I'm also getting segfaults when I tried to do this:
std::cout << "Bond ATOMS : "
<< (*current_molecule).my_bonds[0].particle_1->name
<< std::endl
Again, current_molecule is a pointer to a s_molecule structure, which contains arrays of structures, which in turn either directly have vars or are pointers. I can't get these multiple layers of indirection to work. Suggestions on fixing these segfaults.
FYI I'm compiling on Linux Centos 5.4 with g++ and using a custom makefile system.
#sbi Thanks for the good advice! I believe you are right -- the assignment overloaded operator is unnecessary and should be scrapped.
I've followed the approach of commenting out stuff and am very confused. Basically in the function that passes the pointer to my particular molecule to the main function to print, I can see all the data in that molecule (bonds, particles, name, etc) perfectly, printing with cout's.
Once I pass it to the main as a ptr, if I use that ptr with an iterator I get a segfault. In other words. Also for some reason the bond data (which I can freely print in my funct that returns to the pointer) also segfaults if I try to print it, even if I use the [] to index the vector of bonds (which works for the particle vector).
That's the best info I can give for now.
A wild guess: Are you using shared libraries. I remember having difficulties passing STL-containers back and forth across shared library boundaries.
Jason (OP) was asked in a comment by David Rodríguez:
Are you returning a pointer to a local variable?
Jason answered:
No its a ptr to a class variable. The class is very much in existence (it contains the function that returns the molecule).
Unless you're talking of a true class variable (qualified as static), the fact that the class exists doesn't have much to do with it. Instances of a class exist, and they might have ceased to exist even if you just called a function on them.
As such, the question is:
Does the instance of the class that returned the pointer current_molecule still exist?
Or is current_molecule qualified as static, i.e. being a true class variable?
If the answer to both questions is "no", you're in Undefined County.
At this point, it becomes very important that you post source code that can be used by us here to actually reproduce the problem; it might well be located in source you aren't showing us.
Again, this issue was answered here:
Weird Pointer issue in C++
by DeadMG. Sorry for the double post.