Why is a temporary char** argument illegal?

up vote
16
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 it into

char * p[2] = "a", "b";
f(p);

and that went fine [would have been also fine with just char * p].

Why can't I create an array of pointers on the fly like the following?

f("a", "b");

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

add a commentÂ |Â

up vote
16
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 it into

char * p[2] = "a", "b";
f(p);

and that went fine [would have been also fine with just char * p].

Why can't I create an array of pointers on the fly like the following?

f("a", "b");

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

add a commentÂ |Â

up vote
16
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 it into

char * p[2] = "a", "b";
f(p);

and that went fine [would have been also fine with just char * p].

Why can't I create an array of pointers on the fly like the following?

f("a", "b");

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

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 it into

char * p[2] = "a", "b";
f(p);

and that went fine [would have been also fine with just char * p].

Why can't I create an array of pointers on the fly like the following?

f("a", "b");

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

edited 17 mins ago

Peter Mortensen

13.2k1983111

edited 17 mins ago

Peter Mortensen

13.2k1983111

edited 17 mins ago

Peter Mortensen

13.2k1983111

asked 5 hours ago

CIsForCookies

6,55111443

asked 5 hours ago

CIsForCookies

6,55111443

asked 5 hours ago

CIsForCookies

6,55111443

add a commentÂ |Â

3 Answers
3

active

oldest

votes

up vote
24
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 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

add a commentÂ |Â

up vote
11
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 5 hours ago

dasblinkenlight

603k547591176

add a commentÂ |Â

up vote
3
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 5 hours ago

answered 5 hours 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
5 hours ago

1

@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
5 hours ago

add a commentÂ |Â

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
24
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 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

add a commentÂ |Â

up vote
24
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 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

add a commentÂ |Â

up vote
24
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 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

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 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

edited 5 hours ago

answered 5 hours ago

dbush

86.7k1092125

answered 5 hours ago

dbush

86.7k1092125

answered 5 hours ago

dbush

86.7k1092125

add a commentÂ |Â

up vote
11
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 5 hours ago

dasblinkenlight

603k547591176

add a commentÂ |Â

up vote
11
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 5 hours ago

dasblinkenlight

603k547591176

add a commentÂ |Â

up vote
11
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 5 hours ago

dasblinkenlight

603k547591176

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 5 hours ago

dasblinkenlight

603k547591176

answered 5 hours ago

dasblinkenlight

603k547591176

answered 5 hours ago

dasblinkenlight

603k547591176

answered 5 hours ago

dasblinkenlight

603k547591176

add a commentÂ |Â

up vote
3
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 5 hours ago

answered 5 hours 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
5 hours ago

1

@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
5 hours ago

add a commentÂ |Â

up vote
3
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 5 hours ago

answered 5 hours 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
5 hours ago

1

@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
5 hours ago

add a commentÂ |Â

up vote
3
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 5 hours ago

answered 5 hours 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 5 hours ago

answered 5 hours ago

haccks

84.8k20125216

edited 5 hours ago

answered 5 hours ago

haccks

84.8k20125216

answered 5 hours ago

haccks

84.8k20125216

answered 5 hours 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
5 hours ago

1

@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
5 hours 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
5 hours ago

1

@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
5 hours ago

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

add a commentÂ |Â

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