Related
I am trying to understand how to use reference parameters. There are several examples in my text, however they are too complicated for me to understand why and how to use them.
How and why would you want to use a reference? What would happen if you didn't make the parameter a reference, but instead left the & off?
For example, what's the difference between these functions:
int doSomething(int& a, int& b);
int doSomething(int a, int b);
I understand that reference variables are used in order to change a formal->reference, which then allows a two-way exchange of parameters. However, that is the extent of my knowledge, and a more concrete example would be of much help.
Think of a reference as an alias. When you invoke something on a reference, you're really invoking it on the object to which the reference refers.
int i;
int& j = i; // j is an alias to i
j = 5; // same as i = 5
When it comes to functions, consider:
void foo(int i)
{
i = 5;
}
Above, int i is a value and the argument passed is passed by value. That means if we say:
int x = 2;
foo(x);
i will be a copy of x. Thus setting i to 5 has no effect on x, because it's the copy of x being changed. However, if we make i a reference:
void foo(int& i) // i is an alias for a variable
{
i = 5;
}
Then saying foo(x) no longer makes a copy of x; i is x. So if we say foo(x), inside the function i = 5; is exactly the same as x = 5;, and x changes.
Hopefully that clarifies a bit.
Why is this important? When you program, you never want to copy and paste code. You want to make a function that does one task and it does it well. Whenever that task needs to be performed, you use that function.
So let's say we want to swap two variables. That looks something like this:
int x, y;
// swap:
int temp = x; // store the value of x
x = y; // make x equal to y
y = temp; // make y equal to the old value of x
Okay, great. We want to make this a function, because: swap(x, y); is much easier to read. So, let's try this:
void swap(int x, int y)
{
int temp = x;
x = y;
y = temp;
}
This won't work! The problem is that this is swapping copies of two variables. That is:
int a, b;
swap(a, b); // hm, x and y are copies of a and b...a and b remain unchanged
In C, where references do not exist, the solution was to pass the address of these variables; that is, use pointers*:
void swap(int* x, int* y)
{
int temp = *x;
*x = *y;
*y = temp;
}
int a, b;
swap(&a, &b);
This works well. However, it's a bit clumsy to use, and actually a bit unsafe. swap(nullptr, nullptr), swaps two nothings and dereferences null pointers...undefined behavior! Fixable with some checks:
void swap(int* x, int* y)
{
if (x == nullptr || y == nullptr)
return; // one is null; this is a meaningless operation
int temp = *x;
*x = *y;
*y = temp;
}
But looks how clumsy our code has gotten. C++ introduces references to solve this problem. If we can just alias a variable, we get the code we were looking for:
void swap(int& x, int& y)
{
int temp = x;
x = y;
y = temp;
}
int a, b;
swap(a, b); // inside, x and y are really a and b
Both easy to use, and safe. (We can't accidentally pass in a null, there are no null references.) This works because the swap happening inside the function is really happening on the variables being aliased outside the function.
(Note, never write a swap function. :) One already exists in the header <algorithm>, and it's templated to work with any type.)
Another use is to remove that copy that happens when you call a function. Consider we have a data type that's very big. Copying this object takes a lot of time, and we'd like to avoid that:
struct big_data
{ char data[9999999]; }; // big!
void do_something(big_data data);
big_data d;
do_something(d); // ouch, making a copy of all that data :<
However, all we really need is an alias to the variable, so let's indicate that. (Again, back in C we'd pass the address of our big data type, solving the copying problem but introducing clumsiness.):
void do_something(big_data& data);
big_data d;
do_something(d); // no copies at all! data aliases d within the function
This is why you'll hear it said you should pass things by reference all the time, unless they are primitive types. (Because internally passing an alias is probably done with a pointer, like in C. For small objects it's just faster to make the copy then worry about pointers.)
Keep in mind you should be const-correct. This means if your function doesn't modify the parameter, mark it as const. If do_something above only looked at but didn't change data, we'd mark it as const:
void do_something(const big_data& data); // alias a big_data, and don't change it
We avoid the copy and we say "hey, we won't be modifying this." This has other side effects (with things like temporary variables), but you shouldn't worry about that now.
In contrast, our swap function cannot be const, because we are indeed modifying the aliases.
Hope this clarifies some more.
*Rough pointers tutorial:
A pointer is a variable that holds the address of another variable. For example:
int i; // normal int
int* p; // points to an integer (is not an integer!)
p = &i; // &i means "address of i". p is pointing to i
*p = 2; // *p means "dereference p". that is, this goes to the int
// pointed to by p (i), and sets it to 2.
So, if you've seen the pointer-version swap function, we pass the address of the variables we want to swap, and then we do the swap, dereferencing to get and set values.
Lets take a simple example of a function named increment which increments its argument. Consider:
void increment(int input) {
input++;
}
which will not work as the change takes place on the copy of the argument passed to the function on the actual parameter. So
int i = 1;
std::cout<<i<<" ";
increment(i);
std::cout<<i<<" ";
will produce 1 1 as output.
To make the function work on the actual parameter passed we pass its reference to the function as:
void increment(int &input) { // note the &
input++;
}
the change made to input inside the function is actually being made to the actual parameter. This will produce the expected output of 1 2
GMan's answer gives you the lowdown on references. I just wanted to show you a very basic function that must use references: swap, which swaps two variables. Here it is for ints (as you requested):
// changes to a & b hold when the function exits
void swap(int& a, int& b) {
int tmp = a;
a = b;
b = tmp;
}
// changes to a & b are local to swap_noref and will go away when the function exits
void swap_noref(int a, int b) {
int tmp = a;
a = b;
b = tmp;
}
// changes swap_ptr makes to the variables pointed to by pa & pb
// are visible outside swap_ptr, but changes to pa and pb won't be visible
void swap_ptr(int *pa, int *pb) {
int tmp = *pa;
*pa = *pb;
*pb = tmp;
}
int main() {
int x = 17;
int y = 42;
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap can alter x & y
swap(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_noref can't alter x or y
swap_noref(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_ptr can alter x & y
swap_ptr(&x,&y);
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
}
There is a cleverer swap implementation for ints that doesn't need a temporary. However, here I care more about clear than clever.
Without references (or pointers), swap_noref cannot alter the variables passed to it, which means it simply cannot work. swap_ptr can alter variables, but it uses pointers, which are messy (when references won't quite cut it, however, pointers can do the job). swap is the simplest overall.
On Pointers
Pointers let you do some of the same things as references. However, pointers put more responsibility on the programmer to manage them and the memory they point to (a topic called "memory management"–but don't worry about it for now). As a consequence, references should be your preferred tool for now.
Think of variables as names bound to boxes that store a value. Constants are names bound directly to values. Both map names to values, but the value of constants can't be changed. While the value held in a box can change, the binding of name to box can't, which is why a reference cannot be changed to refer to a different variable.
Two basic operations on variables are getting the current value (done simply by using the variable's name) and assigning a new value (the assignment operator, '='). Values are stored in memory (the box holding a value is simply a contiguous region of memory). For example,
int a = 17;
results in something like (note: in the following, "foo # 0xDEADBEEF" stands for a variable with name "foo" stored at address "0xDEADBEEF". Memory addresses have been made up):
____
a # 0x1000: | 17 |
----
Everything stored in memory has a starting address, so there's one more operation: get the address of the value ("&" is the address-of operator). A pointer is a variable that stores an address.
int *pa = &a;
results in:
______ ____
pa # 0x10A0: |0x1000| ------> # 0x1000: | 17 |
------ ----
Note that a pointer simply stores a memory address, so it doesn't have access to the name of what it points to. In fact, pointers can point to things without names, but that's a topic for another day.
There are a few operations on pointers. You can dereference a pointer (the "*" operator), which gives you the data the pointer points to. Dereferencing is the opposite of getting the address: *&a is the same box as a, &*pa is the same value as pa, and *pa is the same box as a. In particular, pa in the example holds 0x1000; * pa means "the int in memory at location pa", or "the int in memory at location 0x1000". "a" is also "the int at memory location 0x1000". Other operation on pointers are addition and subtraction, but that's also a topic for another day.
// Passes in mutable references of a and b.
int doSomething(int& a, int& b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 5,6
Alternatively,
// Passes in copied values of a and b.
int doSomething(int a, int b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 0,6
Or the const version:
// Passes in const references a and b.
int doSomething(const int &a, const int &b) {
a = 5; // COMPILE ERROR, cannot assign to const reference.
cout << "1: " << b; // prints 1: 6
}
a = 0;
b = 6;
doSomething(a, b);
References are used to pass locations of variables, so they don't need to be copied on the stack to the new function.
A simple pair of examples which you can run online.
The first uses a normal function, and the second uses references:
Example 1 (no reference)
Example 2 (reference)
Edit - here's the source code incase you don't like links:
Example 1
using namespace std;
void foo(int y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 1
}
Example 2
using namespace std;
void foo(int & y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 2
}
I don't know if this is the most basic, but here goes...
typedef int Element;
typedef std::list<Element> ElementList;
// Defined elsewhere.
bool CanReadElement(void);
Element ReadSingleElement(void);
int ReadElementsIntoList(int count, ElementList& elems)
{
int elemsRead = 0;
while(elemsRead < count && CanReadElement())
elems.push_back(ReadSingleElement());
return count;
}
Here we use a reference to pass our list of elements into ReadElementsIntoList(). This way, the function loads the elements right into the list. If we didn't use a reference, then elems would be a copy of the passed-in list, which would have the elements added to it, but then elems would be discarded when the function returns.
This works both ways. In the case of count, we don't make it a reference, because we don't want to modify the count passed in, instead returning the number of elements read. This allows the calling code to compare the number of elements actually read to the requested number; if they don't match, then CanReadElement() must have returned false, and immediately trying to read some more would likely fail. If they match, then maybe count was less than the number of elements available, and a further read would be appropriate. Finally, if ReadElementsIntoList() needed to modify count internally, it could do so without mucking up the caller.
How about by metaphor: Say your function counts beans in a jar. It needs the jar of beans and you need to know the result which can't be the return value (for any number of reasons). You could send it the jar and the variable value, but you'll never know if or what it changes the value to. Instead, you need to send it that variable via a return addressed envelope, so it can put the value in that and know it's written the result to the value at said address.
Correct me if I'm wrong, but a reference is only a dereferenced pointer, or?
The difference to a pointer is, that you can't easily commit a NULL.
I am trying to understand how to use reference parameters. There are several examples in my text, however they are too complicated for me to understand why and how to use them.
How and why would you want to use a reference? What would happen if you didn't make the parameter a reference, but instead left the & off?
For example, what's the difference between these functions:
int doSomething(int& a, int& b);
int doSomething(int a, int b);
I understand that reference variables are used in order to change a formal->reference, which then allows a two-way exchange of parameters. However, that is the extent of my knowledge, and a more concrete example would be of much help.
Think of a reference as an alias. When you invoke something on a reference, you're really invoking it on the object to which the reference refers.
int i;
int& j = i; // j is an alias to i
j = 5; // same as i = 5
When it comes to functions, consider:
void foo(int i)
{
i = 5;
}
Above, int i is a value and the argument passed is passed by value. That means if we say:
int x = 2;
foo(x);
i will be a copy of x. Thus setting i to 5 has no effect on x, because it's the copy of x being changed. However, if we make i a reference:
void foo(int& i) // i is an alias for a variable
{
i = 5;
}
Then saying foo(x) no longer makes a copy of x; i is x. So if we say foo(x), inside the function i = 5; is exactly the same as x = 5;, and x changes.
Hopefully that clarifies a bit.
Why is this important? When you program, you never want to copy and paste code. You want to make a function that does one task and it does it well. Whenever that task needs to be performed, you use that function.
So let's say we want to swap two variables. That looks something like this:
int x, y;
// swap:
int temp = x; // store the value of x
x = y; // make x equal to y
y = temp; // make y equal to the old value of x
Okay, great. We want to make this a function, because: swap(x, y); is much easier to read. So, let's try this:
void swap(int x, int y)
{
int temp = x;
x = y;
y = temp;
}
This won't work! The problem is that this is swapping copies of two variables. That is:
int a, b;
swap(a, b); // hm, x and y are copies of a and b...a and b remain unchanged
In C, where references do not exist, the solution was to pass the address of these variables; that is, use pointers*:
void swap(int* x, int* y)
{
int temp = *x;
*x = *y;
*y = temp;
}
int a, b;
swap(&a, &b);
This works well. However, it's a bit clumsy to use, and actually a bit unsafe. swap(nullptr, nullptr), swaps two nothings and dereferences null pointers...undefined behavior! Fixable with some checks:
void swap(int* x, int* y)
{
if (x == nullptr || y == nullptr)
return; // one is null; this is a meaningless operation
int temp = *x;
*x = *y;
*y = temp;
}
But looks how clumsy our code has gotten. C++ introduces references to solve this problem. If we can just alias a variable, we get the code we were looking for:
void swap(int& x, int& y)
{
int temp = x;
x = y;
y = temp;
}
int a, b;
swap(a, b); // inside, x and y are really a and b
Both easy to use, and safe. (We can't accidentally pass in a null, there are no null references.) This works because the swap happening inside the function is really happening on the variables being aliased outside the function.
(Note, never write a swap function. :) One already exists in the header <algorithm>, and it's templated to work with any type.)
Another use is to remove that copy that happens when you call a function. Consider we have a data type that's very big. Copying this object takes a lot of time, and we'd like to avoid that:
struct big_data
{ char data[9999999]; }; // big!
void do_something(big_data data);
big_data d;
do_something(d); // ouch, making a copy of all that data :<
However, all we really need is an alias to the variable, so let's indicate that. (Again, back in C we'd pass the address of our big data type, solving the copying problem but introducing clumsiness.):
void do_something(big_data& data);
big_data d;
do_something(d); // no copies at all! data aliases d within the function
This is why you'll hear it said you should pass things by reference all the time, unless they are primitive types. (Because internally passing an alias is probably done with a pointer, like in C. For small objects it's just faster to make the copy then worry about pointers.)
Keep in mind you should be const-correct. This means if your function doesn't modify the parameter, mark it as const. If do_something above only looked at but didn't change data, we'd mark it as const:
void do_something(const big_data& data); // alias a big_data, and don't change it
We avoid the copy and we say "hey, we won't be modifying this." This has other side effects (with things like temporary variables), but you shouldn't worry about that now.
In contrast, our swap function cannot be const, because we are indeed modifying the aliases.
Hope this clarifies some more.
*Rough pointers tutorial:
A pointer is a variable that holds the address of another variable. For example:
int i; // normal int
int* p; // points to an integer (is not an integer!)
p = &i; // &i means "address of i". p is pointing to i
*p = 2; // *p means "dereference p". that is, this goes to the int
// pointed to by p (i), and sets it to 2.
So, if you've seen the pointer-version swap function, we pass the address of the variables we want to swap, and then we do the swap, dereferencing to get and set values.
Lets take a simple example of a function named increment which increments its argument. Consider:
void increment(int input) {
input++;
}
which will not work as the change takes place on the copy of the argument passed to the function on the actual parameter. So
int i = 1;
std::cout<<i<<" ";
increment(i);
std::cout<<i<<" ";
will produce 1 1 as output.
To make the function work on the actual parameter passed we pass its reference to the function as:
void increment(int &input) { // note the &
input++;
}
the change made to input inside the function is actually being made to the actual parameter. This will produce the expected output of 1 2
GMan's answer gives you the lowdown on references. I just wanted to show you a very basic function that must use references: swap, which swaps two variables. Here it is for ints (as you requested):
// changes to a & b hold when the function exits
void swap(int& a, int& b) {
int tmp = a;
a = b;
b = tmp;
}
// changes to a & b are local to swap_noref and will go away when the function exits
void swap_noref(int a, int b) {
int tmp = a;
a = b;
b = tmp;
}
// changes swap_ptr makes to the variables pointed to by pa & pb
// are visible outside swap_ptr, but changes to pa and pb won't be visible
void swap_ptr(int *pa, int *pb) {
int tmp = *pa;
*pa = *pb;
*pb = tmp;
}
int main() {
int x = 17;
int y = 42;
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap can alter x & y
swap(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_noref can't alter x or y
swap_noref(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_ptr can alter x & y
swap_ptr(&x,&y);
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
}
There is a cleverer swap implementation for ints that doesn't need a temporary. However, here I care more about clear than clever.
Without references (or pointers), swap_noref cannot alter the variables passed to it, which means it simply cannot work. swap_ptr can alter variables, but it uses pointers, which are messy (when references won't quite cut it, however, pointers can do the job). swap is the simplest overall.
On Pointers
Pointers let you do some of the same things as references. However, pointers put more responsibility on the programmer to manage them and the memory they point to (a topic called "memory management"–but don't worry about it for now). As a consequence, references should be your preferred tool for now.
Think of variables as names bound to boxes that store a value. Constants are names bound directly to values. Both map names to values, but the value of constants can't be changed. While the value held in a box can change, the binding of name to box can't, which is why a reference cannot be changed to refer to a different variable.
Two basic operations on variables are getting the current value (done simply by using the variable's name) and assigning a new value (the assignment operator, '='). Values are stored in memory (the box holding a value is simply a contiguous region of memory). For example,
int a = 17;
results in something like (note: in the following, "foo # 0xDEADBEEF" stands for a variable with name "foo" stored at address "0xDEADBEEF". Memory addresses have been made up):
____
a # 0x1000: | 17 |
----
Everything stored in memory has a starting address, so there's one more operation: get the address of the value ("&" is the address-of operator). A pointer is a variable that stores an address.
int *pa = &a;
results in:
______ ____
pa # 0x10A0: |0x1000| ------> # 0x1000: | 17 |
------ ----
Note that a pointer simply stores a memory address, so it doesn't have access to the name of what it points to. In fact, pointers can point to things without names, but that's a topic for another day.
There are a few operations on pointers. You can dereference a pointer (the "*" operator), which gives you the data the pointer points to. Dereferencing is the opposite of getting the address: *&a is the same box as a, &*pa is the same value as pa, and *pa is the same box as a. In particular, pa in the example holds 0x1000; * pa means "the int in memory at location pa", or "the int in memory at location 0x1000". "a" is also "the int at memory location 0x1000". Other operation on pointers are addition and subtraction, but that's also a topic for another day.
// Passes in mutable references of a and b.
int doSomething(int& a, int& b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 5,6
Alternatively,
// Passes in copied values of a and b.
int doSomething(int a, int b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 0,6
Or the const version:
// Passes in const references a and b.
int doSomething(const int &a, const int &b) {
a = 5; // COMPILE ERROR, cannot assign to const reference.
cout << "1: " << b; // prints 1: 6
}
a = 0;
b = 6;
doSomething(a, b);
References are used to pass locations of variables, so they don't need to be copied on the stack to the new function.
A simple pair of examples which you can run online.
The first uses a normal function, and the second uses references:
Example 1 (no reference)
Example 2 (reference)
Edit - here's the source code incase you don't like links:
Example 1
using namespace std;
void foo(int y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 1
}
Example 2
using namespace std;
void foo(int & y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 2
}
I don't know if this is the most basic, but here goes...
typedef int Element;
typedef std::list<Element> ElementList;
// Defined elsewhere.
bool CanReadElement(void);
Element ReadSingleElement(void);
int ReadElementsIntoList(int count, ElementList& elems)
{
int elemsRead = 0;
while(elemsRead < count && CanReadElement())
elems.push_back(ReadSingleElement());
return count;
}
Here we use a reference to pass our list of elements into ReadElementsIntoList(). This way, the function loads the elements right into the list. If we didn't use a reference, then elems would be a copy of the passed-in list, which would have the elements added to it, but then elems would be discarded when the function returns.
This works both ways. In the case of count, we don't make it a reference, because we don't want to modify the count passed in, instead returning the number of elements read. This allows the calling code to compare the number of elements actually read to the requested number; if they don't match, then CanReadElement() must have returned false, and immediately trying to read some more would likely fail. If they match, then maybe count was less than the number of elements available, and a further read would be appropriate. Finally, if ReadElementsIntoList() needed to modify count internally, it could do so without mucking up the caller.
How about by metaphor: Say your function counts beans in a jar. It needs the jar of beans and you need to know the result which can't be the return value (for any number of reasons). You could send it the jar and the variable value, but you'll never know if or what it changes the value to. Instead, you need to send it that variable via a return addressed envelope, so it can put the value in that and know it's written the result to the value at said address.
Correct me if I'm wrong, but a reference is only a dereferenced pointer, or?
The difference to a pointer is, that you can't easily commit a NULL.
I am trying to do a simple library where the object is a point on the xy-axis.
I want to be able to use literals like this:
Point a = (3,4);
where (3,4) is a point literal.
I read about user defined literals, but (as I understood) this seems to be impossible.
May be "(3,4)"_P is possible as I understand it.
However, I found on this page interesting use of user defined literals as follows:
#include <iostream>
#include <complex>
int main()
{
using namespace std::complex_literals;
std::complex<double> c = 1.0 + 1i;
std::cout << "abs" << c << " = " << abs(c) << '\n';
}
I can under stand the part 1i as a user defined literal, but not the whole thing 1.0 + 1i.
What I am missing, and what is the nearest possible way of getting a literal similar to (x,y) without using ".
As Some programmer dude shows, the best way is to use uniform initialization.
However, just for the fun of it, you can (sort of) do this with User Defined Literals. My idea is to to have 2 literals for each coordinate and overload operator+ between them to create the point.
Remember, this is just for fun, don't use this in a real code:
struct Px { int x; };
struct Py { int y; };
struct Point {
int x;
int y;
};
constexpr auto operator""_px(unsigned long long x) -> Px { return Px{(int)x}; }
constexpr auto operator""_py(unsigned long long y) -> Py { return Py{(int)y}; }
constexpr auto operator+(Px x, Py y) -> Point { return Point{x.x, y.y}; }
then you can have:
auto p = 3_px + 4_py; // p is deduced to type `Point`
Of course this is just a rough framework. Read this great article to learn more about UDLs. You would need to deal with the narrowing conversion in a better way and propper use namespaces to make it a better solution.
As a bonus, you could also use operator, to create a syntax more appropriate to what you had in mind. But, don't do this, as overloading operator, is just evil:
auto operator,(Px x, Py y) -> Point { return Point{x.x, y.y}; }
auto p = (2_px, 1_py); // p is deduced to type `Point`
You can't make up literals on your own, only create suffixes for literals. Like the shown 1i or the standard language f as in 1.0f. (See e.g. this user-defined literal reference for more information.)
What you can to is to use uniform initialization doing something like
Point a = { 3, 4 }; // note the use of curly-braces
Depending on what Point is you might need to add a suitable constructor to make it work.
You have 3 options
Point p = { 1,2 };
Point p2{ 1,2 };
Point p3(1,2);
Consider the following struct:
struct Vector4D
{
union
{
double components[4];
struct { double x, y, z, t; } Endpoint;
};
};
It seems to me that I have seen something similar in WinApi's IPAddress struct. The idea is to give me the possibility to use the array components both by index and by name, for example:
Vector4D v;
v.components[2] = 3.0;
ASSERT(v.Endpoint.z == 3.0) //let's ignore precision issues for now
In the C++ standard there is a guarantee that there will be no "empty" space at the beginning of a POD-struct, that is, the element x will be situated right in the beginnig of the Endpoint struct. Good so far. But I don't seem to find any guarantees that there will be no empty space or padding, if you will, between x and y, or y and z, etc. I haven't checked out the C99 standard though.
The problem is that if there is an empty space between Endpoint struct elements, then the idea will not work.
Questions:
Am I right that there indeed is no guarantee that this will work either in C or C++.
Will this practically work on any known implementation? In other words, do you know of any implementation where this doesn't work?
Is there any standard(I mean not compiler-specific) way to express the same idea? Maybe the C++0x alignment features might help?
By the way, this isn't something I am doing in production code, don't worry, just curious. Thanks in advance.
yes
depends on the alignment needs of the architecture and the compilers strategy
no, but you could make a object wrapper (but you will end up with .z() instead of just .z)
Most compilers should support squashing a structure using a pragma or an attribute. #pragma pack for example.
You can circumvent any memory alignment issues by having references to each element of the array, as long as you declare the array before the references in the class to ensure they point to valid data. Having said that I doubt alignment would be an issue with doubles, but could be for other types (float on 64bit arch perhaps?)
#include <iostream>
using namespace std;
struct Vector4D
{
Vector4D() : components(), x(components[0]), y(components[1]), z(components[2]), t(components[3]) { }
double components[4];
double& x;
double& y;
double& z;
double& t;
};
int main()
{
Vector4D v;
v.components[0] = 3.0;
v.components[1] = 1.0;
v.components[2] = 4.0;
v.components[3] = 15.0;
cout << v.x << endl;
cout << v.y << endl;
cout << v.z << endl;
cout << v.t << endl;
}
Hope this helps.
When it comes to the standard, there are two problems with it:
It is unspecified what happens when writing to an element in a union and reading from another, see the C standard 6.2.6.1 and K.1
The standard does not guarantee the layout of the struct match that of the layout of the array, see the C standard 6.7.2.1.10 for details.
Having said this, in practice this will work on normal compilers. In fact, this kind of code is widely spread and is often used to reinterpret values of one type into values of another type.
Padding bytes will not cause an issue as all variables are of type double. The compiler will treat Vector4D as a double array. That means, v.Endpoint.z is essentially the same as v[2].
I am trying to understand how to use reference parameters. There are several examples in my text, however they are too complicated for me to understand why and how to use them.
How and why would you want to use a reference? What would happen if you didn't make the parameter a reference, but instead left the & off?
For example, what's the difference between these functions:
int doSomething(int& a, int& b);
int doSomething(int a, int b);
I understand that reference variables are used in order to change a formal->reference, which then allows a two-way exchange of parameters. However, that is the extent of my knowledge, and a more concrete example would be of much help.
Think of a reference as an alias. When you invoke something on a reference, you're really invoking it on the object to which the reference refers.
int i;
int& j = i; // j is an alias to i
j = 5; // same as i = 5
When it comes to functions, consider:
void foo(int i)
{
i = 5;
}
Above, int i is a value and the argument passed is passed by value. That means if we say:
int x = 2;
foo(x);
i will be a copy of x. Thus setting i to 5 has no effect on x, because it's the copy of x being changed. However, if we make i a reference:
void foo(int& i) // i is an alias for a variable
{
i = 5;
}
Then saying foo(x) no longer makes a copy of x; i is x. So if we say foo(x), inside the function i = 5; is exactly the same as x = 5;, and x changes.
Hopefully that clarifies a bit.
Why is this important? When you program, you never want to copy and paste code. You want to make a function that does one task and it does it well. Whenever that task needs to be performed, you use that function.
So let's say we want to swap two variables. That looks something like this:
int x, y;
// swap:
int temp = x; // store the value of x
x = y; // make x equal to y
y = temp; // make y equal to the old value of x
Okay, great. We want to make this a function, because: swap(x, y); is much easier to read. So, let's try this:
void swap(int x, int y)
{
int temp = x;
x = y;
y = temp;
}
This won't work! The problem is that this is swapping copies of two variables. That is:
int a, b;
swap(a, b); // hm, x and y are copies of a and b...a and b remain unchanged
In C, where references do not exist, the solution was to pass the address of these variables; that is, use pointers*:
void swap(int* x, int* y)
{
int temp = *x;
*x = *y;
*y = temp;
}
int a, b;
swap(&a, &b);
This works well. However, it's a bit clumsy to use, and actually a bit unsafe. swap(nullptr, nullptr), swaps two nothings and dereferences null pointers...undefined behavior! Fixable with some checks:
void swap(int* x, int* y)
{
if (x == nullptr || y == nullptr)
return; // one is null; this is a meaningless operation
int temp = *x;
*x = *y;
*y = temp;
}
But looks how clumsy our code has gotten. C++ introduces references to solve this problem. If we can just alias a variable, we get the code we were looking for:
void swap(int& x, int& y)
{
int temp = x;
x = y;
y = temp;
}
int a, b;
swap(a, b); // inside, x and y are really a and b
Both easy to use, and safe. (We can't accidentally pass in a null, there are no null references.) This works because the swap happening inside the function is really happening on the variables being aliased outside the function.
(Note, never write a swap function. :) One already exists in the header <algorithm>, and it's templated to work with any type.)
Another use is to remove that copy that happens when you call a function. Consider we have a data type that's very big. Copying this object takes a lot of time, and we'd like to avoid that:
struct big_data
{ char data[9999999]; }; // big!
void do_something(big_data data);
big_data d;
do_something(d); // ouch, making a copy of all that data :<
However, all we really need is an alias to the variable, so let's indicate that. (Again, back in C we'd pass the address of our big data type, solving the copying problem but introducing clumsiness.):
void do_something(big_data& data);
big_data d;
do_something(d); // no copies at all! data aliases d within the function
This is why you'll hear it said you should pass things by reference all the time, unless they are primitive types. (Because internally passing an alias is probably done with a pointer, like in C. For small objects it's just faster to make the copy then worry about pointers.)
Keep in mind you should be const-correct. This means if your function doesn't modify the parameter, mark it as const. If do_something above only looked at but didn't change data, we'd mark it as const:
void do_something(const big_data& data); // alias a big_data, and don't change it
We avoid the copy and we say "hey, we won't be modifying this." This has other side effects (with things like temporary variables), but you shouldn't worry about that now.
In contrast, our swap function cannot be const, because we are indeed modifying the aliases.
Hope this clarifies some more.
*Rough pointers tutorial:
A pointer is a variable that holds the address of another variable. For example:
int i; // normal int
int* p; // points to an integer (is not an integer!)
p = &i; // &i means "address of i". p is pointing to i
*p = 2; // *p means "dereference p". that is, this goes to the int
// pointed to by p (i), and sets it to 2.
So, if you've seen the pointer-version swap function, we pass the address of the variables we want to swap, and then we do the swap, dereferencing to get and set values.
Lets take a simple example of a function named increment which increments its argument. Consider:
void increment(int input) {
input++;
}
which will not work as the change takes place on the copy of the argument passed to the function on the actual parameter. So
int i = 1;
std::cout<<i<<" ";
increment(i);
std::cout<<i<<" ";
will produce 1 1 as output.
To make the function work on the actual parameter passed we pass its reference to the function as:
void increment(int &input) { // note the &
input++;
}
the change made to input inside the function is actually being made to the actual parameter. This will produce the expected output of 1 2
GMan's answer gives you the lowdown on references. I just wanted to show you a very basic function that must use references: swap, which swaps two variables. Here it is for ints (as you requested):
// changes to a & b hold when the function exits
void swap(int& a, int& b) {
int tmp = a;
a = b;
b = tmp;
}
// changes to a & b are local to swap_noref and will go away when the function exits
void swap_noref(int a, int b) {
int tmp = a;
a = b;
b = tmp;
}
// changes swap_ptr makes to the variables pointed to by pa & pb
// are visible outside swap_ptr, but changes to pa and pb won't be visible
void swap_ptr(int *pa, int *pb) {
int tmp = *pa;
*pa = *pb;
*pb = tmp;
}
int main() {
int x = 17;
int y = 42;
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap can alter x & y
swap(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_noref can't alter x or y
swap_noref(x,y);
// next line will print "x: 42; y: 17"
std::cout << "x: " << x << "; y: " << y << std::endl
// swap_ptr can alter x & y
swap_ptr(&x,&y);
// next line will print "x: 17; y: 42"
std::cout << "x: " << x << "; y: " << y << std::endl
}
There is a cleverer swap implementation for ints that doesn't need a temporary. However, here I care more about clear than clever.
Without references (or pointers), swap_noref cannot alter the variables passed to it, which means it simply cannot work. swap_ptr can alter variables, but it uses pointers, which are messy (when references won't quite cut it, however, pointers can do the job). swap is the simplest overall.
On Pointers
Pointers let you do some of the same things as references. However, pointers put more responsibility on the programmer to manage them and the memory they point to (a topic called "memory management"–but don't worry about it for now). As a consequence, references should be your preferred tool for now.
Think of variables as names bound to boxes that store a value. Constants are names bound directly to values. Both map names to values, but the value of constants can't be changed. While the value held in a box can change, the binding of name to box can't, which is why a reference cannot be changed to refer to a different variable.
Two basic operations on variables are getting the current value (done simply by using the variable's name) and assigning a new value (the assignment operator, '='). Values are stored in memory (the box holding a value is simply a contiguous region of memory). For example,
int a = 17;
results in something like (note: in the following, "foo # 0xDEADBEEF" stands for a variable with name "foo" stored at address "0xDEADBEEF". Memory addresses have been made up):
____
a # 0x1000: | 17 |
----
Everything stored in memory has a starting address, so there's one more operation: get the address of the value ("&" is the address-of operator). A pointer is a variable that stores an address.
int *pa = &a;
results in:
______ ____
pa # 0x10A0: |0x1000| ------> # 0x1000: | 17 |
------ ----
Note that a pointer simply stores a memory address, so it doesn't have access to the name of what it points to. In fact, pointers can point to things without names, but that's a topic for another day.
There are a few operations on pointers. You can dereference a pointer (the "*" operator), which gives you the data the pointer points to. Dereferencing is the opposite of getting the address: *&a is the same box as a, &*pa is the same value as pa, and *pa is the same box as a. In particular, pa in the example holds 0x1000; * pa means "the int in memory at location pa", or "the int in memory at location 0x1000". "a" is also "the int at memory location 0x1000". Other operation on pointers are addition and subtraction, but that's also a topic for another day.
// Passes in mutable references of a and b.
int doSomething(int& a, int& b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 5,6
Alternatively,
// Passes in copied values of a and b.
int doSomething(int a, int b) {
a = 5;
cout << "1: " << a << b; // prints 1: 5,6
}
a = 0;
b = 6;
doSomething(a, b);
cout << "2: " << a << ", " << b; // prints 2: 0,6
Or the const version:
// Passes in const references a and b.
int doSomething(const int &a, const int &b) {
a = 5; // COMPILE ERROR, cannot assign to const reference.
cout << "1: " << b; // prints 1: 6
}
a = 0;
b = 6;
doSomething(a, b);
References are used to pass locations of variables, so they don't need to be copied on the stack to the new function.
A simple pair of examples which you can run online.
The first uses a normal function, and the second uses references:
Example 1 (no reference)
Example 2 (reference)
Edit - here's the source code incase you don't like links:
Example 1
using namespace std;
void foo(int y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 1
}
Example 2
using namespace std;
void foo(int & y){
y=2;
}
int main(){
int x=1;
foo(x);
cout<<x;//outputs 2
}
I don't know if this is the most basic, but here goes...
typedef int Element;
typedef std::list<Element> ElementList;
// Defined elsewhere.
bool CanReadElement(void);
Element ReadSingleElement(void);
int ReadElementsIntoList(int count, ElementList& elems)
{
int elemsRead = 0;
while(elemsRead < count && CanReadElement())
elems.push_back(ReadSingleElement());
return count;
}
Here we use a reference to pass our list of elements into ReadElementsIntoList(). This way, the function loads the elements right into the list. If we didn't use a reference, then elems would be a copy of the passed-in list, which would have the elements added to it, but then elems would be discarded when the function returns.
This works both ways. In the case of count, we don't make it a reference, because we don't want to modify the count passed in, instead returning the number of elements read. This allows the calling code to compare the number of elements actually read to the requested number; if they don't match, then CanReadElement() must have returned false, and immediately trying to read some more would likely fail. If they match, then maybe count was less than the number of elements available, and a further read would be appropriate. Finally, if ReadElementsIntoList() needed to modify count internally, it could do so without mucking up the caller.
How about by metaphor: Say your function counts beans in a jar. It needs the jar of beans and you need to know the result which can't be the return value (for any number of reasons). You could send it the jar and the variable value, but you'll never know if or what it changes the value to. Instead, you need to send it that variable via a return addressed envelope, so it can put the value in that and know it's written the result to the value at said address.
Correct me if I'm wrong, but a reference is only a dereferenced pointer, or?
The difference to a pointer is, that you can't easily commit a NULL.