C Language Pointers Const Pointers
Example
Single Pointers
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Pointer to an
intThe pointer can point to different integers and the
int's can be changed through the pointer. This sample of code assigns b to point toint bthen changesb's value to100.int b; int* p; p = &b; /* OK */ *p = 100; /* OK */ -
Pointer to a
const intThe pointer can point to different integers but the
int's value can't be changed through the pointer.int b; const int* p; p = &b; /* OK */ *p = 100; /* Compiler Error */ -
constpointer tointThe pointer can only point to one
intbut theint's value can be changed through the pointer.int a, b; int* const p = &b; /* OK as initialisation, no assignment */ *p = 100; /* OK */ p = &a; /* Compiler Error */
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constpointer toconst intThe pointer can only point to one
intand theintcan not be changed through the pointer.int a, b; const int* const p = &b; /* OK as initialisation, no assignment */ p = &a; /* Compiler Error */ *p = 100; /* Compiler Error */
Pointer to Pointer
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Pointer to a pointer to an
intThis code assigns the address of
p1to the to double pointerp(which then points toint* p1(which points toint)).Then changes
p1to point toint a. Then changes the value of a to be 100.void f1(void) { int a, b; int *p1; int **p; p1 = &b; /* OK */ p = &p1; /* OK */ *p = &a; /* OK */ **p = 100; /* OK */ }
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Pointer to pointer to a
const intvoid f2(void) { int b; const int *p1; const int **p; p = &p1; /* OK */ *p = &b; /* OK */ **p = 100; /* error: assignment of read-only location ‘**p’ */ } -
Pointer to
constpointer to anintvoid f3(void) { int b; int *p1; int * const *p; p = &p1; /* OK */ *p = &b; /* error: assignment of read-only location ‘*p’ */ **p = 100; /* OK */ } -
constpointer to pointer tointvoid f4(void) { int b; int *p1; int ** const p = &p1; /* OK as initialisation, not assignment */ p = &p1; /* error: assignment of read-only variable ‘p’ */ *p = &b; /* OK */ **p = 100; /* OK */ } -
Pointer to
constpointer toconst intvoid f5(void) { int b; const int *p1; const int * const *p; p = &p1; /* OK */ *p = &b; /* error: assignment of read-only location ‘*p’ */ **p = 100; /* error: assignment of read-only location ‘**p’ */ } -
constpointer to pointer toconst intvoid f6(void) { int b; const int *p1; const int ** const p = &p1; /* OK as initialisation, not assignment */ p = &p1; /* error: assignment of read-only variable ‘p’ */ *p = &b; /* OK */ **p = 100; /* error: assignment of read-only location ‘**p’ */ } -
constpointer toconstpointer tointvoid f7(void) { int b; int *p1; int * const * const p = &p1; /* OK as initialisation, not assignment */ p = &p1; /* error: assignment of read-only variable ‘p’ */ *p = &b; /* error: assignment of read-only location ‘*p’ */ **p = 100; /* OK */ }
