glVertex3f and glVertex2fx displaying different results

up vote
0
down vote

favorite

I suspect it has to do with my conversion code:

vector3::operator float*() const

{

    // x, y, z are member floats

    float arr[3];

    arr[0] = x;

    arr[1] = y;

    arr[2] = z;

    return arr;

}

Then in another class I do:

glBegin(GL_POLYGON);

    glVertex3fv(origin); // wrong result

    //glVertex3f(origin.x, origin.y, origin.z); // good

    //glVertex3f(0.0, 0.0, 0.0); // also good

    glVertex3f(1.0, 0.0, 0.0);

    glVertex3f(1.0, 1.0, 0.0);

    glVertex3f(0.0, 1.0, 0.0);

glEnd();

The problem is that the rectangle is stretched very far. I suspect it is because of the way I am passing the argument.

asked Nov 21 at 5:44

jinenofu

New contributor

Hi @jinenofu please provide a Minimum, Complete and Verifiable example see: stackoverflow.com/help/mcve
– AdaRaider
Nov 21 at 5:51

4

float arr[3]; is a local variable in a method. You return a pointer to local data. The data go out of scope (are lost) after the function has terminated. Read a basic C++ or C tutorial.
– Rabbid76
Nov 21 at 5:55

What is origin? glVertex3fv expects a pointer to 3 contiguous GLfloats. Are you sure that origin has a type which grants this? What type does it actually have?
– Scheff
Nov 21 at 7:18

I assume, you want to call glVertex for your DIY type vector3? How about an "overload"? E.g. void glVertex(const vector3 &v) { glVertex3f(v.x, v.y, v.z); } Then, you could do e.g. vector3 origin(1, 2, 3); glVertex(origin);. (I made some assumptions about your code but I guess you got the idea.)
– Scheff
Nov 21 at 7:21

This is a C++ problem, not an OpenGL one. You're returning an address to a temporary. UB. Without seeing the sources of vector3 it's hard to answer definitely, but the recommendation I'd give is the same - ditch your own class and just use GLM. There are apparently more ways to write a vec3 badly than to do it correctly.
– Bartek Banachewicz
Nov 21 at 8:49

|
show 2 more comments

up vote
0
down vote

favorite

I suspect it has to do with my conversion code:

vector3::operator float*() const

{

    // x, y, z are member floats

    float arr[3];

    arr[0] = x;

    arr[1] = y;

    arr[2] = z;

    return arr;

}

Then in another class I do:

glBegin(GL_POLYGON);

    glVertex3fv(origin); // wrong result

    //glVertex3f(origin.x, origin.y, origin.z); // good

    //glVertex3f(0.0, 0.0, 0.0); // also good

    glVertex3f(1.0, 0.0, 0.0);

    glVertex3f(1.0, 1.0, 0.0);

    glVertex3f(0.0, 1.0, 0.0);

glEnd();

The problem is that the rectangle is stretched very far. I suspect it is because of the way I am passing the argument.

asked Nov 21 at 5:44

jinenofu

New contributor

Hi @jinenofu please provide a Minimum, Complete and Verifiable example see: stackoverflow.com/help/mcve
– AdaRaider
Nov 21 at 5:51

4

float arr[3]; is a local variable in a method. You return a pointer to local data. The data go out of scope (are lost) after the function has terminated. Read a basic C++ or C tutorial.
– Rabbid76
Nov 21 at 5:55

What is origin? glVertex3fv expects a pointer to 3 contiguous GLfloats. Are you sure that origin has a type which grants this? What type does it actually have?
– Scheff
Nov 21 at 7:18

I assume, you want to call glVertex for your DIY type vector3? How about an "overload"? E.g. void glVertex(const vector3 &v) { glVertex3f(v.x, v.y, v.z); } Then, you could do e.g. vector3 origin(1, 2, 3); glVertex(origin);. (I made some assumptions about your code but I guess you got the idea.)
– Scheff
Nov 21 at 7:21

This is a C++ problem, not an OpenGL one. You're returning an address to a temporary. UB. Without seeing the sources of vector3 it's hard to answer definitely, but the recommendation I'd give is the same - ditch your own class and just use GLM. There are apparently more ways to write a vec3 badly than to do it correctly.
– Bartek Banachewicz
Nov 21 at 8:49

|
show 2 more comments

up vote
0
down vote

favorite

I suspect it has to do with my conversion code:

vector3::operator float*() const

{

    // x, y, z are member floats

    float arr[3];

    arr[0] = x;

    arr[1] = y;

    arr[2] = z;

    return arr;

}

Then in another class I do:

glBegin(GL_POLYGON);

    glVertex3fv(origin); // wrong result

    //glVertex3f(origin.x, origin.y, origin.z); // good

    //glVertex3f(0.0, 0.0, 0.0); // also good

    glVertex3f(1.0, 0.0, 0.0);

    glVertex3f(1.0, 1.0, 0.0);

    glVertex3f(0.0, 1.0, 0.0);

glEnd();

The problem is that the rectangle is stretched very far. I suspect it is because of the way I am passing the argument.

asked Nov 21 at 5:44

jinenofu

New contributor

I suspect it has to do with my conversion code:

vector3::operator float*() const

{

    // x, y, z are member floats

    float arr[3];

    arr[0] = x;

    arr[1] = y;

    arr[2] = z;

    return arr;

}

Then in another class I do:

glBegin(GL_POLYGON);

    glVertex3fv(origin); // wrong result

    //glVertex3f(origin.x, origin.y, origin.z); // good

    //glVertex3f(0.0, 0.0, 0.0); // also good

    glVertex3f(1.0, 0.0, 0.0);

    glVertex3f(1.0, 1.0, 0.0);

    glVertex3f(0.0, 1.0, 0.0);

glEnd();

The problem is that the rectangle is stretched very far. I suspect it is because of the way I am passing the argument.

c++ arrays opengl casting

asked Nov 21 at 5:44

jinenofu

New contributor

asked Nov 21 at 5:44

jinenofu

New contributor

asked Nov 21 at 5:44

jinenofu

New contributor

asked Nov 21 at 5:44

jinenofu

asked Nov 21 at 5:44

jinenofu

New contributor

jinenofu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

Hi @jinenofu please provide a Minimum, Complete and Verifiable example see: stackoverflow.com/help/mcve
– AdaRaider
Nov 21 at 5:51

4

float arr[3]; is a local variable in a method. You return a pointer to local data. The data go out of scope (are lost) after the function has terminated. Read a basic C++ or C tutorial.
– Rabbid76
Nov 21 at 5:55

What is origin? glVertex3fv expects a pointer to 3 contiguous GLfloats. Are you sure that origin has a type which grants this? What type does it actually have?
– Scheff
Nov 21 at 7:18

I assume, you want to call glVertex for your DIY type vector3? How about an "overload"? E.g. void glVertex(const vector3 &v) { glVertex3f(v.x, v.y, v.z); } Then, you could do e.g. vector3 origin(1, 2, 3); glVertex(origin);. (I made some assumptions about your code but I guess you got the idea.)
– Scheff
Nov 21 at 7:21

This is a C++ problem, not an OpenGL one. You're returning an address to a temporary. UB. Without seeing the sources of vector3 it's hard to answer definitely, but the recommendation I'd give is the same - ditch your own class and just use GLM. There are apparently more ways to write a vec3 badly than to do it correctly.
– Bartek Banachewicz
Nov 21 at 8:49

|
show 2 more comments

Hi @jinenofu please provide a Minimum, Complete and Verifiable example see: stackoverflow.com/help/mcve
– AdaRaider
Nov 21 at 5:51

4

float arr[3]; is a local variable in a method. You return a pointer to local data. The data go out of scope (are lost) after the function has terminated. Read a basic C++ or C tutorial.
– Rabbid76
Nov 21 at 5:55

What is origin? glVertex3fv expects a pointer to 3 contiguous GLfloats. Are you sure that origin has a type which grants this? What type does it actually have?
– Scheff
Nov 21 at 7:18

I assume, you want to call glVertex for your DIY type vector3? How about an "overload"? E.g. void glVertex(const vector3 &v) { glVertex3f(v.x, v.y, v.z); } Then, you could do e.g. vector3 origin(1, 2, 3); glVertex(origin);. (I made some assumptions about your code but I guess you got the idea.)
– Scheff
Nov 21 at 7:21

This is a C++ problem, not an OpenGL one. You're returning an address to a temporary. UB. Without seeing the sources of vector3 it's hard to answer definitely, but the recommendation I'd give is the same - ditch your own class and just use GLM. There are apparently more ways to write a vec3 badly than to do it correctly.
– Bartek Banachewicz
Nov 21 at 8:49

Hi @jinenofu please provide a Minimum, Complete and Verifiable example see: stackoverflow.com/help/mcve
– AdaRaider
Nov 21 at 5:51

float arr[3]; is a local variable in a method. You return a pointer to local data. The data go out of scope (are lost) after the function has terminated. Read a basic C++ or C tutorial.
– Rabbid76
Nov 21 at 5:55

What is origin? glVertex3fv expects a pointer to 3 contiguous GLfloats. Are you sure that origin has a type which grants this? What type does it actually have?
– Scheff
Nov 21 at 7:18

I assume, you want to call glVertex for your DIY type vector3? How about an "overload"? E.g. void glVertex(const vector3 &v) { glVertex3f(v.x, v.y, v.z); } Then, you could do e.g. vector3 origin(1, 2, 3); glVertex(origin);. (I made some assumptions about your code but I guess you got the idea.)
– Scheff
Nov 21 at 7:21

This is a C++ problem, not an OpenGL one. You're returning an address to a temporary. UB. Without seeing the sources of vector3 it's hard to answer definitely, but the recommendation I'd give is the same - ditch your own class and just use GLM. There are apparently more ways to write a vec3 badly than to do it correctly.
– Bartek Banachewicz
Nov 21 at 8:49

|
show 2 more comments

1 Answer
1

active

oldest

votes

up vote
2
down vote

You can't do this in C/C++ legally:

vector3::operator float*() const

{

    float arr[3];

    // ...

    return arr;

}

It invokes undefined behavior. When operator float*() returns, arr goes out of scope and the pointer returned becomes invalid.

Consider yourself lucky, that you've got no nasal demons ;-)

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

up vote
2
down vote

You can't do this in C/C++ legally:

vector3::operator float*() const

{

    float arr[3];

    // ...

    return arr;

}

It invokes undefined behavior. When operator float*() returns, arr goes out of scope and the pointer returned becomes invalid.

Consider yourself lucky, that you've got no nasal demons ;-)

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

add a comment |

up vote
2
down vote

You can't do this in C/C++ legally:

vector3::operator float*() const

{

    float arr[3];

    // ...

    return arr;

}

It invokes undefined behavior. When operator float*() returns, arr goes out of scope and the pointer returned becomes invalid.

Consider yourself lucky, that you've got no nasal demons ;-)

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

add a comment |

up vote
2
down vote

You can't do this in C/C++ legally:

vector3::operator float*() const

{

    float arr[3];

    // ...

    return arr;

}

It invokes undefined behavior. When operator float*() returns, arr goes out of scope and the pointer returned becomes invalid.

Consider yourself lucky, that you've got no nasal demons ;-)

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

You can't do this in C/C++ legally:

vector3::operator float*() const

{

    float arr[3];

    // ...

    return arr;

}

It invokes undefined behavior. When operator float*() returns, arr goes out of scope and the pointer returned becomes invalid.

Consider yourself lucky, that you've got no nasal demons ;-)

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

edited Nov 21 at 14:53

answered Nov 21 at 11:33

datenwolf

131k9128231

answered Nov 21 at 11:33

datenwolf

131k9128231

answered Nov 21 at 11:33

datenwolf

131k9128231

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

add a comment |

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

So what should I do?
– jinenofu
Nov 22 at 0:05

For one (that's OpenGL related): Don't use immediate mode (anything that makes use of glBegin/glEnd). It's outdated for over 20 years. But if you really want to go down that road and insist on using the …v versions, then put your x, y, z members at contiguous locations in your class, or use an array there and return a pointer to that. However this will require, that you don't use that pointer, when the class instance goes out of scope.
– datenwolf
Nov 22 at 7:33

@jinenofu: You could also introduce a new class member function glvertex that internally calls glVertex.
– datenwolf
Nov 22 at 7:34

add a comment |

jinenofu is a new contributor. Be nice, and check out our Code of Conduct.

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