warning: taking address of temporary

[daniell]

/*
Triangle.cpp
*/

// Use a pure virtual function.

#include <iostream>
#include <cstring>
using namespace std;

// A class for two-dimensional objects.
class TwoDShape {
// these are private
double width;
double height;

// add a name field
char name[20];
public:

// Default constructor.
TwoDShape() {
width = height = 0.0;
strcpy(name, "unknown");
}

// Constructor for TwoDShape.
TwoDShape(doubl e w, double h, char *n) {
width = w;
height = h;
strcpy(name, n);
}

// Construct object with equal width and height.
TwoDShape(doubl e x, char *n) {
width = height = x;
strcpy(name, n);
}

void showDim() {
cout << "Width and height are " << width << " and " << height <<
"\n";
}

// accessor functions
double getWidth() { return width; }
double getHeight() { return height; }
void setWidth(double w) { width = w; }
void setHeight(doubl e h) { height = h; }
char *getName() { return name; }

// area() is now a pure virtual function
virtual double area() = 0;

};

// Triangle is derived from TwoDShape.
class Triangle : public TwoDShape {
char style[20]; //now private
public:

/* A default constructor. This automatically invokes
the default constructor of TwoDShape. */
Triangle() {
strcpy(style, "unknown");
}

// Constructor with three parameters.
Triangle(char *str, double w, double h)
: TwoDShape(w, h, "triangle") {
strcpy(style, str);
}

// Construct an isosceles triangle.
Triangle(double x) : TwoDShape(x, x, "triangle") {
strcpy(style, "isosceles" );
}

// This now overrides area() declared in TwoDShape.
double area() {
return getWidth() * getHeight() / 2;
}

void showStyle() {
cout << "Triangle is " << style << "\n";
}
};

int main() {
// declare an array of pointers to TwoDShape objects.
TwoDShape *shapes[4];

shapes[0] = &Triangle("righ t", 8.0, 12.0);
shapes[1] = &Triangle(3. 0);
shapes[2] = &Triangle(4. 0);
shapes[3] = &Triangle(7. 0);

for(int i=0; i < 2; i++) {
cout << "object is " <<
shapes[i]->getName() << "\n";

cout << "Area is " <<
shapes[i]->area() << "\n";

cout << "\n";
}

return 0;
}

Hi, I'm a beginning programmer. Above is the code that is giving me
the error. I have talked with a professor about this issue. The
error happens on the lines beginning "shapes[0] = ... "

The professor says, because Triangle was created without a variable
they are temporary objects. Which means they will be destroyed and
the pointer becomes a dangling pointer.

I edited the code and just created the objects and gave them a
variable to be stored in. i.e. "Triangle t1("right", 4, 5);" Then I
just do "shapes[0] = &t1;" When I recompile, I get no warnings and it
runs as expected.

My question: Is there a way so that the temporary objects are not
destroyed? Seems like a waste of memory to create a variable outside
of the array that will always be referenced through the array.

[dave_mikesell]

On May 8, 10:58 am, dani...@digital filing.com wrote:

<snip>

My question: Is there a way so that the temporary objects are not
destroyed? Seems like a waste of memory to create a variable outside
of the array that will always be referenced through the array.

For the purpose of this example, create the Triangles with new.

shapes[0] = new Triangle(...);

shapes[0] now holds the address of the created Triangle.

If you're writing an application that does something like this, you
might want to look at std::vector and boost::shared_p tr.

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-05-08 16:58, da*****@digital filing.com wrote:

/*
Triangle.cpp
*/

// Use a pure virtual function.

#include <iostream>
#include <cstring>
using namespace std;

// A class for two-dimensional objects.
class TwoDShape {
// these are private
double width;
double height;

// add a name field
char name[20];
public:

// Default constructor.
TwoDShape() {
width = height = 0.0;
strcpy(name, "unknown");
}

// Constructor for TwoDShape.
TwoDShape(doubl e w, double h, char *n) {
width = w;
height = h;
strcpy(name, n);
}

// Construct object with equal width and height.
TwoDShape(doubl e x, char *n) {
width = height = x;
strcpy(name, n);
}

void showDim() {
cout << "Width and height are " << width << " and " << height <<
"\n";
}

// accessor functions
double getWidth() { return width; }
double getHeight() { return height; }
void setWidth(double w) { width = w; }
void setHeight(doubl e h) { height = h; }
char *getName() { return name; }

// area() is now a pure virtual function
virtual double area() = 0;

};

// Triangle is derived from TwoDShape.
class Triangle : public TwoDShape {
char style[20]; //now private
public:

/* A default constructor. This automatically invokes
the default constructor of TwoDShape. */
Triangle() {
strcpy(style, "unknown");
}

// Constructor with three parameters.
Triangle(char *str, double w, double h)
: TwoDShape(w, h, "triangle") {
strcpy(style, str);
}

// Construct an isosceles triangle.
Triangle(double x) : TwoDShape(x, x, "triangle") {
strcpy(style, "isosceles" );
}

// This now overrides area() declared in TwoDShape.
double area() {
return getWidth() * getHeight() / 2;
}

void showStyle() {
cout << "Triangle is " << style << "\n";
}
};

int main() {
// declare an array of pointers to TwoDShape objects.
TwoDShape *shapes[4];

shapes[0] = &Triangle("righ t", 8.0, 12.0);
shapes[1] = &Triangle(3. 0);
shapes[2] = &Triangle(4. 0);
shapes[3] = &Triangle(7. 0);

for(int i=0; i < 2; i++) {
cout << "object is " <<
shapes[i]->getName() << "\n";

cout << "Area is " <<
shapes[i]->area() << "\n";

cout << "\n";
}

return 0;
}

Hi, I'm a beginning programmer. Above is the code that is giving me
the error. I have talked with a professor about this issue. The
error happens on the lines beginning "shapes[0] = ... "

The professor says, because Triangle was created without a variable
they are temporary objects. Which means they will be destroyed and
the pointer becomes a dangling pointer.

I edited the code and just created the objects and gave them a
variable to be stored in. i.e. "Triangle t1("right", 4, 5);" Then I
just do "shapes[0] = &t1;" When I recompile, I get no warnings and it
runs as expected.

My question: Is there a way so that the temporary objects are not
destroyed? Seems like a waste of memory to create a variable outside
of the array that will always be referenced through the array.

The simplest (as in requires the least change in the code) way to do
this is to use new to allocate the Triangles on the heap (dynamic
memory). Objects created with new will continue to exist until they are
explicitly deleted (even if you no longer have a pointer or reference to
them!). This means that when you no longer need an object created with
new you must delete them or your program will use up all memory.

To create a Triangle using new do this:

shapes[0] = new Triangle("right ", 8.0, 12.0);

And when you are done with it you use delete to delete them:

delete shapes[0];

I guess that you have not discusses dynamic memory in class, beware that
there are some problems with it if you are not careful and it is
therefore generally a good idea not to use it, but sometimes you have to.

--
Erik Wikström

[daniell]

On May 8, 11:28 am, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-05-08 16:58, dani...@digital filing.com wrote:

/*
Triangle.cpp
*/

// Use a pure virtual function.

#include <iostream>
#include <cstring>
using namespace std;

// A class for two-dimensional objects.
class TwoDShape {
// these are private
double width;
double height;

// add a name field
char name[20];
public:

// Default constructor.
TwoDShape() {
width = height = 0.0;
strcpy(name, "unknown");
}

// Constructor for TwoDShape.
TwoDShape(doubl e w, double h, char *n) {
width = w;
height = h;
strcpy(name, n);
}

// Construct object with equal width and height.
TwoDShape(doubl e x, char *n) {
width = height = x;
strcpy(name, n);
}

void showDim() {
cout << "Width and height are " << width << " and " << height <<
"\n";
}

// accessor functions
double getWidth() { return width; }
double getHeight() { return height; }
void setWidth(double w) { width = w; }
void setHeight(doubl e h) { height = h; }
char *getName() { return name; }

// area() is now a pure virtual function
virtual double area() = 0;

};

// Triangle is derived from TwoDShape.
class Triangle : public TwoDShape {
char style[20]; //now private
public:

/* A default constructor. This automatically invokes
the default constructor of TwoDShape. */
Triangle() {
strcpy(style, "unknown");
}

// Constructor with three parameters.
Triangle(char *str, double w, double h)
: TwoDShape(w, h, "triangle") {
strcpy(style, str);
}

// Construct an isosceles triangle.
Triangle(double x) : TwoDShape(x, x, "triangle") {
strcpy(style, "isosceles" );
}

// This now overrides area() declared in TwoDShape.
double area() {
return getWidth() * getHeight() / 2;
}

void showStyle() {
cout << "Triangle is " << style << "\n";
}
};

int main() {
// declare an array of pointers to TwoDShape objects.
TwoDShape *shapes[4];

shapes[0] = &Triangle("righ t", 8.0, 12.0);
shapes[1] = &Triangle(3. 0);
shapes[2] = &Triangle(4. 0);
shapes[3] = &Triangle(7. 0);

for(int i=0; i < 2; i++) {
cout << "object is " <<
shapes[i]->getName() << "\n";

cout << "Area is " <<
shapes[i]->area() << "\n";

cout << "\n";
}

return 0;
}

Hi, I'm a beginning programmer. Above is the code that is giving me
the error. I have talked with a professor about this issue. The
error happens on the lines beginning "shapes[0] = ... "

The professor says, because Triangle was created without a variable
they are temporary objects. Which means they will be destroyed and
the pointer becomes a dangling pointer.

I edited the code and just created the objects and gave them a
variable to be stored in. i.e. "Triangle t1("right", 4, 5);" Then I
just do "shapes[0] = &t1;" When I recompile, I get no warnings and it
runs as expected.

My question: Is there a way so that the temporary objects are not
destroyed? Seems like a waste of memory to create a variable outside
of the array that will always be referenced through the array.

The simplest (as in requires the least change in the code) way to do
this is to use new to allocate the Triangles on the heap (dynamic
memory). Objects created with new will continue to exist until they are
explicitly deleted (even if you no longer have a pointer or reference to
them!). This means that when you no longer need an object created with
new you must delete them or your program will use up all memory.

To create a Triangle using new do this:

shapes[0] = new Triangle("right ", 8.0, 12.0);

And when you are done with it you use delete to delete them:

delete shapes[0];

I guess that you have not discusses dynamic memory in class, beware that
there are some problems with it if you are not careful and it is
therefore generally a good idea not to use it, but sometimes you have to.

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Yes the suggestion of using the new keyword solves the issue. Thanks
for the response.

I am actually taking a refresher on C++ and dynamic memory has not
been discussed yet. But I do recall the dangers of it. Again thanks
for the help!