Re: == comparison of string literals, and their usage
Le 07/04/2019 à 14:23, bauss via Digitalmars-d-learn a écrit :
On Saturday, 6 April 2019 at 19:47:14 UTC, lithium iodate wrote:
On Saturday, 6 April 2019 at 15:35:22 UTC, diniz wrote:
So, I still could store and use and compare string pointers myself [1], and
get valid results, meaning: pointer equality implies (literal) string
equality. Or am I wrong? The point is, the parser, operating on an array of
prescanned lexemes, will constantly check whether a valid lexeme is present
simply by checking the lexeme "class". I don't want that to be a real string
comp, too expesensive and for no gain.
[1] As in the second comp of your example:
void main()
{
auto c2 = "one" == "two";
auto c1 = "one".ptr is "two".ptr;
}
Not quite. D-strings strictly consist of pointer *and* length, so you need to
compare the .length properties as well to correctly conclude that the strings
equal. You can concisely do that in one go by simply `is` comparing the array
references as in
string a = "hello";
string b = a;
assert(a is b);
assert(a[] is b[]);
Of course, if the strings are never sliced, you can just compare the pointers
and be done, just make sure to document how it operates. Depending on the
circumstances I'd throw in some asserts that do actual strings comparison to
verify the program logic.
To add onto this.
Here is an example why it's important to compare the length as well:
string a = "hello";
string b = a[0 .. 3];
assert(a.ptr == b.ptr);
assert(a.length != b.length);
Thank you! Very clear :-).
--
diniz {la vita e estranj}
Re: == comparison of string literals, and their usage
On Saturday, 6 April 2019 at 19:47:14 UTC, lithium iodate wrote:
On Saturday, 6 April 2019 at 15:35:22 UTC, diniz wrote:
So, I still could store and use and compare string pointers
myself [1], and get valid results, meaning: pointer equality
implies (literal) string equality. Or am I wrong? The point
is, the parser, operating on an array of prescanned lexemes,
will constantly check whether a valid lexeme is present simply
by checking the lexeme "class". I don't want that to be a real
string comp, too expesensive and for no gain.
[1] As in the second comp of your example:
void main()
{
auto c2 = "one" == "two";
auto c1 = "one".ptr is "two".ptr;
}
Not quite. D-strings strictly consist of pointer *and* length,
so you need to compare the .length properties as well to
correctly conclude that the strings equal. You can concisely do
that in one go by simply `is` comparing the array references as
in
string a = "hello";
string b = a;
assert(a is b);
assert(a[] is b[]);
Of course, if the strings are never sliced, you can just
compare the pointers and be done, just make sure to document
how it operates. Depending on the circumstances I'd throw in
some asserts that do actual strings comparison to verify the
program logic.
To add onto this.
Here is an example why it's important to compare the length as
well:
string a = "hello";
string b = a[0 .. 3];
assert(a.ptr == b.ptr);
assert(a.length != b.length);
Re: == comparison of string literals, and their usage
Le 06/04/2019 à 21:47, lithium iodate via Digitalmars-d-learn a écrit :
On Saturday, 6 April 2019 at 15:35:22 UTC, diniz wrote:
So, I still could store and use and compare string pointers myself [1], and
get valid results, meaning: pointer equality implies (literal) string
equality. Or am I wrong? The point is, the parser, operating on an array of
prescanned lexemes, will constantly check whether a valid lexeme is present
simply by checking the lexeme "class". I don't want that to be a real string
comp, too expesensive and for no gain.
[1] As in the second comp of your example:
void main()
{
auto c2 = "one" == "two";
auto c1 = "one".ptr is "two".ptr;
}
Not quite. D-strings strictly consist of pointer *and* length, so you need to
compare the .length properties as well to correctly conclude that the strings
equal. You can concisely do that in one go by simply `is` comparing the array
references as in
string a = "hello";
string b = a;
assert(a is b);
assert(a[] is b[]);
Of course, if the strings are never sliced, you can just compare the pointers
and be done, just make sure to document how it operates. Depending on the
circumstances I'd throw in some asserts that do actual strings comparison to
verify the program logic.
Thank you very much! And yes, properly documenting is also important to me.
--
diniz {la vita e estranj}
Re: == comparison of string literals, and their usage
On Saturday, 6 April 2019 at 15:35:22 UTC, diniz wrote:
So, I still could store and use and compare string pointers
myself [1], and get valid results, meaning: pointer equality
implies (literal) string equality. Or am I wrong? The point is,
the parser, operating on an array of prescanned lexemes, will
constantly check whether a valid lexeme is present simply by
checking the lexeme "class". I don't want that to be a real
string comp, too expesensive and for no gain.
[1] As in the second comp of your example:
void main()
{
auto c2 = "one" == "two";
auto c1 = "one".ptr is "two".ptr;
}
Not quite. D-strings strictly consist of pointer *and* length, so
you need to compare the .length properties as well to correctly
conclude that the strings equal. You can concisely do that in one
go by simply `is` comparing the array references as in
string a = "hello";
string b = a;
assert(a is b);
assert(a[] is b[]);
Of course, if the strings are never sliced, you can just compare
the pointers and be done, just make sure to document how it
operates. Depending on the circumstances I'd throw in some
asserts that do actual strings comparison to verify the program
logic.
Re: == comparison of string literals, and their usage
Le 06/04/2019 à 16:07, AltFunction1 via Digitalmars-d-learn a écrit :
On Friday, 5 April 2019 at 14:49:50 UTC, diniz wrote:
Hello,
Since literal strings are interned (and immutable), can I count on the fact
that they are compared (==) by pointer?
No. "==" performs a full array comparison and "is" is apparently simplified at
compile time. In the compiler there's no notion of string literal as a special
expression. It's always a StringExp. See https://d.godbolt.org/z/K5R6u6.
However you're right to say that literal are not duplicated.
Thank you very much.
So, I still could store and use and compare string pointers myself [1], and get
valid results, meaning: pointer equality implies (literal) string equality. Or
am I wrong? The point is, the parser, operating on an array of prescanned
lexemes, will constantly check whether a valid lexeme is present simply by
checking the lexeme "class". I don't want that to be a real string comp, too
expesensive and for no gain.
[1] As in the second comp of your example:
void main()
{
auto c2 = "one" == "two";
auto c1 = "one".ptr is "two".ptr;
}
--
diniz {la vita e estranj}
Re: == comparison of string literals, and their usage
On Friday, 5 April 2019 at 14:49:50 UTC, diniz wrote: Hello, Since literal strings are interned (and immutable), can I count on the fact that they are compared (==) by pointer? No. "==" performs a full array comparison and "is" is apparently simplified at compile time. In the compiler there's no notion of string literal as a special expression. It's always a StringExp. See https://d.godbolt.org/z/K5R6u6. However you're right to say that literal are not duplicated.
