References, Pointers and Constants: Difference between revisions
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==Simple C++ Data Types== | ==Simple C++ Data Types== | ||
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!style="color:#ffb900; width:120px"|C++ | !style="color:#ffb900; width:120px"|C++ code | ||
!style="color:#ffb900"|What | !style="color:#ffb900"|What it means | ||
!style="color:#ffb900"|When | !style="color:#ffb900"|When to use it | ||
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|char<br> | | | ||
char<br> | |||
int<br> | int<br> | ||
double | double | ||
|Single (scalar) value (character, integer, floating point number) | | | ||
|To store data<br> | Single (scalar) value (character, integer, floating point number) | ||
| | |||
To store data<br> | |||
To pass data to a function by value<br> | To pass data to a function by value<br> | ||
To return data by value | To return data by value | ||
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const char<br> | const char<br> | ||
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To declare a constant -- a value that cannot be changed<br> | To declare a constant -- a value that cannot be changed<br> | ||
Has no effect for passing values to a function or from returning values from a function | Has no effect for passing values to a function or from returning values from a function | ||
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char []<br> | char []<br> | ||
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To store the same type of data together<br> | To store the same type of data together<br> | ||
To pass an array to a function | To pass an array to a function | ||
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const char []<br> | const char []<br> | ||
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To declare an array of constant values (no element in the array can be changed)<br> | To declare an array of constant values (no element in the array can be changed)<br> | ||
const char [] is used to declare a string constant | const char [] is used to declare a string constant | ||
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char *<br> | char *<br> | ||
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To pass a value to a function which the function will set (passing by reference)<br> | To pass a value to a function which the function will set (passing by reference)<br> | ||
To return a dynamically-allocated array from a function.<br> | To return a dynamically-allocated array from a function.<br> | ||
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const char *<br> | const char *<br> | ||
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To return a pointer to a constant value<br> | To return a pointer to a constant value<br> | ||
To pass a pointer to a constant value to a function | To pass a pointer to a constant value to a function | ||
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char &<br> | char &<br> | ||
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To pass a value to a function which will be set by the function (without using a pointer) | To pass a value to a function which will be set by the function (without using a pointer) | ||
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char * &<br> | char * &<br> | ||
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To pass a pointer to a function where the function will set the pointer (either by calling new or by assigning to the pointer | To pass a pointer to a function where the function will set the pointer (either by calling new or by assigning to the pointer | ||
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char * const<br> | char * const<br> | ||
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When you want to point to a specific value. Allows to you to change the data that is pointed to, but you can't change the pointer | When you want to point to a specific value. Allows to you to change the data that is pointed to, but you can't change the pointer | ||
|} | |} | ||
==C++Structures== | ==C++Structures== | ||
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!style="color:#ffb900; width:150px"|C++ | !style="color:#ffb900; width:150px"|C++ code | ||
!style="color:#ffb900"|What | !style="color:#ffb900"|What it means | ||
!style="color:#ffb900"|When | !style="color:#ffb900"|When to use it | ||
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struct somestruct<br> | struct somestruct<br> | ||
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To store different types of data together | To store different types of data together | ||
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struct somestruct<br> | struct somestruct<br> | ||
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To store values in a data structure<br> | To store values in a data structure<br> | ||
'''Not''' to be used to pass structures to a function or to return structures from a function (inefficient!) | '''Not''' to be used to pass structures to a function or to return structures from a function (inefficient!) | ||
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const struct somestruct | const struct somestruct | ||
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When you don't want of the member data values to be changed | When you don't want of the member data values to be changed | ||
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struct somestruct & | struct somestruct & | ||
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To pass a data structure to a function where the function will change some data members of that structure | To pass a data structure to a function where the function will change some data members of that structure | ||
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const struct somestruct & | const struct somestruct & | ||
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To pass a data structure to a function where the function will not or cannot change any data members of that structure | To pass a data structure to a function where the function will not or cannot change any data members of that structure | ||
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struct somestruct [] | struct somestruct [] | ||
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Used to group several identical data structures (with different values) together | Used to group several identical data structures (with different values) together | ||
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const struct somestruct [] | const struct somestruct [] | ||
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When you don't want code to be able to change any data structure values in the array | When you don't want code to be able to change any data structure values in the array | ||
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struct somestruct * | struct somestruct * | ||
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When you want to allocate a single data structure or an array of data structures using ''new''<br> | When you want to allocate a single data structure or an array of data structures using ''new''<br> | ||
When you want to pass a single data structure or an array of data structures to a function, or to return same from a function | When you want to pass a single data structure or an array of data structures to a function, or to return same from a function | ||
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const struct somestruct * | const struct somestruct * | ||
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Same as above, but when you don't want the function that gets called, or the code that called the function, to change any values in any data structure | Same as above, but when you don't want the function that gets called, or the code that called the function, to change any values in any data structure | ||
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struct somestruct * & | struct somestruct * & |
Revision as of 09:16, 14 October 2008
Simple C++ Data Types
C++ code | What it means | When to use it |
---|---|---|
char |
Single (scalar) value (character, integer, floating point number) |
To store data |
const char |
Constant value |
To declare a constant -- a value that cannot be changed |
char [] |
Array (vector) of values |
To store the same type of data together |
const char [] |
Constant array of values |
To declare an array of constant values (no element in the array can be changed) |
char * |
Pointer to a single value or an array of values |
To store a string (char *) |
const char * |
Pointer to a constant value |
When you don't want the value at the pointer to be changed, but you can still change the pointer |
char & |
Reference to a value |
To pass a value to a function which will be set by the function (without using a pointer) |
char * & |
Reference to a pointer |
To pass a pointer to a function where the function will set the pointer (either by calling new or by assigning to the pointer |
char * const |
Constant pointer |
When you want to point to a specific value. Allows to you to change the data that is pointed to, but you can't change the pointer |
C++Structures
C++ code | What it means | When to use it |
---|---|---|
struct somestruct |
Data structure (user-defined) |
To store different types of data together |
struct somestruct |
Data structure value |
To store values in a data structure |
const struct somestruct |
Constant data structure value |
When you don't want of the member data values to be changed |
struct somestruct & |
Reference to a data structure value |
To pass a data structure to a function where the function will change some data members of that structure |
const struct somestruct & |
Constant reference to a data structure value |
To pass a data structure to a function where the function will not or cannot change any data members of that structure |
struct somestruct [] |
Array of data structure values |
Used to group several identical data structures (with different values) together |
const struct somestruct [] |
Array of constant data structure values |
When you don't want code to be able to change any data structure values in the array |
struct somestruct * |
Pointer to a data structure value |
When you want to allocate a single data structure or an array of data structures using new |
const struct somestruct * |
Constant pointer to a data structure value |
Same as above, but when you don't want the function that gets called, or the code that called the function, to change any values in any data structure |
struct somestruct * & |
Reference to a pointer to a data structure value |
When the calling function will be allocating the pointer using new or assigning to the pointer |
Using The References and Pointers: * & . and ->
If & is used in a declaration, then it is defining a reference to a value:
- data-type & variable-name Reference to a value
- const data-type & variable-name Constant reference to a value
If & is used in a statement, then it is behaving as the address-of operator, and creating a pointer to a value:
- &variable-name
If * is used between two variables or constants of type int or double, then it is behaving as the multiplication operator:
- variable-name1 * variable-name2
- constant * variable-name
- variable-name * constant
- constant1 * constant2
If * is used in a declaration, then it is defining a pointer:
- data-type * variable-name Pointer to a value
- const data-type * variable-name Pointer to a constant value
- data-type * const variable-name Constant pointer to a value
- const data-type * const variable-name Constant pointer to a constant value
If * is used in a statement, then it is behaving as the dereferencing operator, and creating a value from a pointer:
- *pointer-variable-name
Using Pointers and References To Structures
If we have a data structure:
- struct somestruct
- {
- char * string_member;
- int int_member;
- double floating_point_member;
- };
We can declare an instance of that structure and initialize (assign values to each member of) the instance:
- somestruct aStruct = { "somestring", 1, 3.14159265 };
We can declare a pointer to a somestruct by declaring a pointer variable and taking the address of the instance:
- somestruct * pStruct = &aStruct;
or we can declare a reference to a somestruct by creating a reference variable:
- somestruct & refStruct = aStruct;
or a constant reference:
- const somestruct & crefStruct = aStruct;
or a pointer to a constant somestruct:
- const somestruct * cpStruct = &aStruct;
or a constant pointer to somestruct:
- somestruct * const pcStruct = &aStruct;
Using any pointer to somestruct (constant or otherwise), to access a member, use the indirect reference operator, ->:
- // read a data member
- cout << pcStruct->string_member << endl;
- cout << cpStruct->int_member << endl;
- cout << pStruct->floating_point_member << endl;
- // write to a data member
- pcStruct->string_member = "Test";
- // ^^^^^ OK, only the pointer is constant, not what it points to.
- // cpStruct->int_member++;
- // ^^^^^^^ Not allowed! cpStruct is a pointer to a constant somestruct.
- pStruct->double_member = 2.0 * PI * radius;
- // ^^^^^ OK, neither the pointer nor what it points to is constant.
We can even declare a reference to a somestruct through a pointer:
- somestruct & refStruct2 = *pcStruct; // dereference the pointers
- somestruct & refStruct3 = *pStruct;
- const somestruct & refStruct4 = *cpStruct; // must be constant reference