Why is temporary char** argument illegal?

up vote
7
down vote

favorite

I have a function f(char **p) and I wanted to call it in the simplest way I could.

I tried char ** p = "a", "b"; f(p); and got:

scalar object requires one element in initializer

so I changed into char * p[2] = "a", "b"; f(p); and that went fine [would have been also fine with just char * p].

My question is, why can't I create an array of pointers on the fly like this:

f("a","b");

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

add a commentÂ |Â

up vote
7
down vote

favorite

I have a function f(char **p) and I wanted to call it in the simplest way I could.

I tried char ** p = "a", "b"; f(p); and got:

scalar object requires one element in initializer

so I changed into char * p[2] = "a", "b"; f(p); and that went fine [would have been also fine with just char * p].

My question is, why can't I create an array of pointers on the fly like this:

f("a","b");

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

add a commentÂ |Â

up vote
7
down vote

favorite

I have a function f(char **p) and I wanted to call it in the simplest way I could.

I tried char ** p = "a", "b"; f(p); and got:

scalar object requires one element in initializer

so I changed into char * p[2] = "a", "b"; f(p); and that went fine [would have been also fine with just char * p].

My question is, why can't I create an array of pointers on the fly like this:

f("a","b");

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

I have a function f(char **p) and I wanted to call it in the simplest way I could.

I tried char ** p = "a", "b"; f(p); and got:

scalar object requires one element in initializer

so I changed into char * p[2] = "a", "b"; f(p); and that went fine [would have been also fine with just char * p].

My question is, why can't I create an array of pointers on the fly like this:

f("a","b");

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

edited 16 mins ago

dbush

86.6k1091124

edited 16 mins ago

dbush

86.6k1091124

edited 16 mins ago

dbush

86.6k1091124

asked 36 mins ago

CIsForCookies

6,50611441

asked 36 mins ago

CIsForCookies

6,50611441

asked 36 mins ago

CIsForCookies

6,50611441

add a commentÂ |Â

3 Answers
3

active

oldest

votes

up vote
9
down vote

accepted

This gives a warning:

char ** p = "a", "b";

because p is not an array.

This is also not legal:

f("a", "b");

since curly braces by themselves are not allowed in an expression (but can be used as an initializer).

It is possible to create an array on the fly like that using a compound literal:

f((char *)"a", "b");

You could also use a compound literal to initialize a temporary:

char ** p = (char *)"a", "b";

Unlike the first statement, this is valid because the literal is an array of type char *[2] and will decay to a char ** which can be used to initialize a variable of this type.

See section 6.5.2.5 of the C standard for more details on compound literals.

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

add a commentÂ |Â

up vote
5
down vote

This declaration char ** p = "a", "b"; triggers a warning because C does not know how to interpret the content of braces:

Declaration type char** suggests it's a single pointer-to-pointer-to-char

Content of specifies two items.

You need to tell C that you are initializing a pointer to pointer with a pointer to the initial element of an anonymous array by specifying a type in front of the braces:

char ** p = (char*)"a", "b"

This construct is called a "compound literal". It could be used in a declaration, but it also works as an expression.

Note: if you are planning to pass an array like that to a function, you need to provide a way to determine the size of the array. One approach is to pass NULL to "terminate" the array, like this:

void f(char **p) 
 int i = 0;
 while (*p) 
 printf("%d:%sn", i++, *p++);
 

int main(void) 
 f((char*)"a", "b", NULL);
 return 0;

Demo.

answered 30 mins ago

dasblinkenlight

603k547591174

add a commentÂ |Â

up vote
2
down vote

C99 and later added compound literals for the exact purpose: creating array and structs on the fly

Example:

struct foo int a; char b[2]; structure;
structure = ((struct foo) 1, 'a', 0);
int *y = (int ) 1, 2, 3;
int *z = (int ) 1;

Apart from std C99 and later, GCC provide this feature as an extension too.

In you case this will work

f((char *)"a","b");

(char *)"a","b" is a compound literal which creates an array of 2 pointers to char on the fly.

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

add a commentÂ |Â

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
9
down vote

accepted

This gives a warning:

char ** p = "a", "b";

because p is not an array.

This is also not legal:

f("a", "b");

since curly braces by themselves are not allowed in an expression (but can be used as an initializer).

It is possible to create an array on the fly like that using a compound literal:

f((char *)"a", "b");

You could also use a compound literal to initialize a temporary:

char ** p = (char *)"a", "b";

Unlike the first statement, this is valid because the literal is an array of type char *[2] and will decay to a char ** which can be used to initialize a variable of this type.

See section 6.5.2.5 of the C standard for more details on compound literals.

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

add a commentÂ |Â

up vote
9
down vote

accepted

This gives a warning:

char ** p = "a", "b";

because p is not an array.

This is also not legal:

f("a", "b");

since curly braces by themselves are not allowed in an expression (but can be used as an initializer).

It is possible to create an array on the fly like that using a compound literal:

f((char *)"a", "b");

You could also use a compound literal to initialize a temporary:

char ** p = (char *)"a", "b";

Unlike the first statement, this is valid because the literal is an array of type char *[2] and will decay to a char ** which can be used to initialize a variable of this type.

See section 6.5.2.5 of the C standard for more details on compound literals.

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

add a commentÂ |Â

up vote
9
down vote

accepted

This gives a warning:

char ** p = "a", "b";

because p is not an array.

This is also not legal:

f("a", "b");

since curly braces by themselves are not allowed in an expression (but can be used as an initializer).

It is possible to create an array on the fly like that using a compound literal:

f((char *)"a", "b");

You could also use a compound literal to initialize a temporary:

char ** p = (char *)"a", "b";

Unlike the first statement, this is valid because the literal is an array of type char *[2] and will decay to a char ** which can be used to initialize a variable of this type.

See section 6.5.2.5 of the C standard for more details on compound literals.

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

This gives a warning:

char ** p = "a", "b";

because p is not an array.

This is also not legal:

f("a", "b");

since curly braces by themselves are not allowed in an expression (but can be used as an initializer).

It is possible to create an array on the fly like that using a compound literal:

f((char *)"a", "b");

You could also use a compound literal to initialize a temporary:

char ** p = (char *)"a", "b";

Unlike the first statement, this is valid because the literal is an array of type char *[2] and will decay to a char ** which can be used to initialize a variable of this type.

See section 6.5.2.5 of the C standard for more details on compound literals.

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

edited 11 mins ago

answered 34 mins ago

dbush

86.6k1091124

answered 34 mins ago

dbush

86.6k1091124

answered 34 mins ago

dbush

86.6k1091124

add a commentÂ |Â

up vote
5
down vote

This declaration char ** p = "a", "b"; triggers a warning because C does not know how to interpret the content of braces:

Declaration type char** suggests it's a single pointer-to-pointer-to-char

Content of specifies two items.

You need to tell C that you are initializing a pointer to pointer with a pointer to the initial element of an anonymous array by specifying a type in front of the braces:

char ** p = (char*)"a", "b"

This construct is called a "compound literal". It could be used in a declaration, but it also works as an expression.

void f(char **p) 
 int i = 0;
 while (*p) 
 printf("%d:%sn", i++, *p++);
 

int main(void) 
 f((char*)"a", "b", NULL);
 return 0;

Demo.

answered 30 mins ago

dasblinkenlight

603k547591174

add a commentÂ |Â

up vote
5
down vote

This declaration char ** p = "a", "b"; triggers a warning because C does not know how to interpret the content of braces:

Declaration type char** suggests it's a single pointer-to-pointer-to-char

Content of specifies two items.

You need to tell C that you are initializing a pointer to pointer with a pointer to the initial element of an anonymous array by specifying a type in front of the braces:

char ** p = (char*)"a", "b"

This construct is called a "compound literal". It could be used in a declaration, but it also works as an expression.

void f(char **p) 
 int i = 0;
 while (*p) 
 printf("%d:%sn", i++, *p++);
 

int main(void) 
 f((char*)"a", "b", NULL);
 return 0;

Demo.

answered 30 mins ago

dasblinkenlight

603k547591174

add a commentÂ |Â

up vote
5
down vote

This declaration char ** p = "a", "b"; triggers a warning because C does not know how to interpret the content of braces:

Declaration type char** suggests it's a single pointer-to-pointer-to-char

Content of specifies two items.

You need to tell C that you are initializing a pointer to pointer with a pointer to the initial element of an anonymous array by specifying a type in front of the braces:

char ** p = (char*)"a", "b"

This construct is called a "compound literal". It could be used in a declaration, but it also works as an expression.

void f(char **p) 
 int i = 0;
 while (*p) 
 printf("%d:%sn", i++, *p++);
 

int main(void) 
 f((char*)"a", "b", NULL);
 return 0;

Demo.

answered 30 mins ago

dasblinkenlight

603k547591174

This declaration char ** p = "a", "b"; triggers a warning because C does not know how to interpret the content of braces:

Declaration type char** suggests it's a single pointer-to-pointer-to-char

Content of specifies two items.

You need to tell C that you are initializing a pointer to pointer with a pointer to the initial element of an anonymous array by specifying a type in front of the braces:

char ** p = (char*)"a", "b"

This construct is called a "compound literal". It could be used in a declaration, but it also works as an expression.

void f(char **p) 
 int i = 0;
 while (*p) 
 printf("%d:%sn", i++, *p++);
 

int main(void) 
 f((char*)"a", "b", NULL);
 return 0;

Demo.

answered 30 mins ago

dasblinkenlight

603k547591174

answered 30 mins ago

dasblinkenlight

603k547591174

answered 30 mins ago

dasblinkenlight

603k547591174

answered 30 mins ago

dasblinkenlight

603k547591174

add a commentÂ |Â

up vote
2
down vote

C99 and later added compound literals for the exact purpose: creating array and structs on the fly

Example:

struct foo int a; char b[2]; structure;
structure = ((struct foo) 1, 'a', 0);
int *y = (int ) 1, 2, 3;
int *z = (int ) 1;

Apart from std C99 and later, GCC provide this feature as an extension too.

In you case this will work

f((char *)"a","b");

(char *)"a","b" is a compound literal which creates an array of 2 pointers to char on the fly.

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

add a commentÂ |Â

up vote
2
down vote

C99 and later added compound literals for the exact purpose: creating array and structs on the fly

Example:

struct foo int a; char b[2]; structure;
structure = ((struct foo) 1, 'a', 0);
int *y = (int ) 1, 2, 3;
int *z = (int ) 1;

Apart from std C99 and later, GCC provide this feature as an extension too.

In you case this will work

f((char *)"a","b");

(char *)"a","b" is a compound literal which creates an array of 2 pointers to char on the fly.

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

add a commentÂ |Â

up vote
2
down vote

C99 and later added compound literals for the exact purpose: creating array and structs on the fly

Example:

struct foo int a; char b[2]; structure;
structure = ((struct foo) 1, 'a', 0);
int *y = (int ) 1, 2, 3;
int *z = (int ) 1;

Apart from std C99 and later, GCC provide this feature as an extension too.

In you case this will work

f((char *)"a","b");

(char *)"a","b" is a compound literal which creates an array of 2 pointers to char on the fly.

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

C99 and later added compound literals for the exact purpose: creating array and structs on the fly

Example:

struct foo int a; char b[2]; structure;
structure = ((struct foo) 1, 'a', 0);
int *y = (int ) 1, 2, 3;
int *z = (int ) 1;

Apart from std C99 and later, GCC provide this feature as an extension too.

In you case this will work

f((char *)"a","b");

(char *)"a","b" is a compound literal which creates an array of 2 pointers to char on the fly.

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

edited 11 mins ago

answered 25 mins ago

haccks

84.8k20125216

answered 25 mins ago

haccks

84.8k20125216

answered 25 mins ago

haccks

84.8k20125216

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

add a commentÂ |Â

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

GCC provide this feature as an extension too - This (f((char *)"a","b");) would not have worked on Clang?
â€“Â CIsForCookies
17 mins ago

@CIsForCookies; Clang also provide the same as an extension. In clang int y = (int ) 1, 2, 3; will work too while GCC will throw an error.
â€“Â haccks
9 mins ago

add a commentÂ |Â

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