result of evaluating block statement in c++
I have the simplest c++ statement declaring a variable "a":
int a = ({ int b = 10; b; });
As recent gcc and clang says, it's a valid statement which really declares variable a having value 10.
The question is: what is this? I know about various types of expressions. I know about various types of statements. But I can't find in a c++ 14 standard any mentioning that "block statement as an expression returns latest inner evaluated expression" or something like this.
Could somebody please point me exact lines of a standard saying that is code line is fully valid?
c++ c++14
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
add a comment |
I have the simplest c++ statement declaring a variable "a":
int a = ({ int b = 10; b; });
As recent gcc and clang says, it's a valid statement which really declares variable a having value 10.
The question is: what is this? I know about various types of expressions. I know about various types of statements. But I can't find in a c++ 14 standard any mentioning that "block statement as an expression returns latest inner evaluated expression" or something like this.
Could somebody please point me exact lines of a standard saying that is code line is fully valid?
c++ c++14
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
2
This is not C++. If you want to be a language lawyer, you need to be-pedantic.
– n.m.
Nov 24 '18 at 11:11
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
A standard conforming way to do something like this would be to use a directly invoked lambda - likeint a = { int b = 10; return b; }();.
– Jesper Juhl
Nov 24 '18 at 11:34
add a comment |
I have the simplest c++ statement declaring a variable "a":
int a = ({ int b = 10; b; });
As recent gcc and clang says, it's a valid statement which really declares variable a having value 10.
The question is: what is this? I know about various types of expressions. I know about various types of statements. But I can't find in a c++ 14 standard any mentioning that "block statement as an expression returns latest inner evaluated expression" or something like this.
Could somebody please point me exact lines of a standard saying that is code line is fully valid?
c++ c++14
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
I have the simplest c++ statement declaring a variable "a":
int a = ({ int b = 10; b; });
As recent gcc and clang says, it's a valid statement which really declares variable a having value 10.
The question is: what is this? I know about various types of expressions. I know about various types of statements. But I can't find in a c++ 14 standard any mentioning that "block statement as an expression returns latest inner evaluated expression" or something like this.
Could somebody please point me exact lines of a standard saying that is code line is fully valid?
c++ c++14
c++ c++14
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
edited Nov 24 '18 at 11:14
Matthieu Brucher
13.5k32140
13.5k32140
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
asked Nov 24 '18 at 11:06
Vyacheslav GrigoryevVyacheslav Grigoryev
1014
1014
2
This is not C++. If you want to be a language lawyer, you need to be-pedantic.
– n.m.
Nov 24 '18 at 11:11
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
A standard conforming way to do something like this would be to use a directly invoked lambda - likeint a = { int b = 10; return b; }();.
– Jesper Juhl
Nov 24 '18 at 11:34
add a comment |
2
This is not C++. If you want to be a language lawyer, you need to be-pedantic.
– n.m.
Nov 24 '18 at 11:11
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
A standard conforming way to do something like this would be to use a directly invoked lambda - likeint a = { int b = 10; return b; }();.
– Jesper Juhl
Nov 24 '18 at 11:34
2
2
This is not C++. If you want to be a language lawyer, you need to be
-pedantic.– n.m.
Nov 24 '18 at 11:11
This is not C++. If you want to be a language lawyer, you need to be
-pedantic.– n.m.
Nov 24 '18 at 11:11
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
A standard conforming way to do something like this would be to use a directly invoked lambda - like
int a = { int b = 10; return b; }(); .– Jesper Juhl
Nov 24 '18 at 11:34
A standard conforming way to do something like this would be to use a directly invoked lambda - like
int a = { int b = 10; return b; }(); .– Jesper Juhl
Nov 24 '18 at 11:34
add a comment |
1 Answer
1
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votes
The question is: what is this?
This is a GNU extension to ISO standard C, an extension that is available also for C++, but likewise is not part of ISO C++.
Citing the GCC Manual: Chapter 6 - Extensions to the C Language Family:
...
These extensions are available in C and Objective-C. Most of them are also available in C++. ...
- Statement Exprs: Putting statements and declarations inside expressions.
Where the latter is explained in detail in GCC Manual: Section 6.1 - Statements and Declarations in Expressions:
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the absolute value of
foo ().
...
As for Clang, the Clang Language Extensions describes that Clang aims to support many GCC extensions: [emhpasis mine]:
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
add a comment |
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1 Answer
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1 Answer
1
active
oldest
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active
oldest
votes
active
oldest
votes
The question is: what is this?
This is a GNU extension to ISO standard C, an extension that is available also for C++, but likewise is not part of ISO C++.
Citing the GCC Manual: Chapter 6 - Extensions to the C Language Family:
...
These extensions are available in C and Objective-C. Most of them are also available in C++. ...
- Statement Exprs: Putting statements and declarations inside expressions.
Where the latter is explained in detail in GCC Manual: Section 6.1 - Statements and Declarations in Expressions:
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the absolute value of
foo ().
...
As for Clang, the Clang Language Extensions describes that Clang aims to support many GCC extensions: [emhpasis mine]:
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
add a comment |
The question is: what is this?
This is a GNU extension to ISO standard C, an extension that is available also for C++, but likewise is not part of ISO C++.
Citing the GCC Manual: Chapter 6 - Extensions to the C Language Family:
...
These extensions are available in C and Objective-C. Most of them are also available in C++. ...
- Statement Exprs: Putting statements and declarations inside expressions.
Where the latter is explained in detail in GCC Manual: Section 6.1 - Statements and Declarations in Expressions:
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the absolute value of
foo ().
...
As for Clang, the Clang Language Extensions describes that Clang aims to support many GCC extensions: [emhpasis mine]:
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
add a comment |
The question is: what is this?
This is a GNU extension to ISO standard C, an extension that is available also for C++, but likewise is not part of ISO C++.
Citing the GCC Manual: Chapter 6 - Extensions to the C Language Family:
...
These extensions are available in C and Objective-C. Most of them are also available in C++. ...
- Statement Exprs: Putting statements and declarations inside expressions.
Where the latter is explained in detail in GCC Manual: Section 6.1 - Statements and Declarations in Expressions:
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the absolute value of
foo ().
...
As for Clang, the Clang Language Extensions describes that Clang aims to support many GCC extensions: [emhpasis mine]:
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
The question is: what is this?
This is a GNU extension to ISO standard C, an extension that is available also for C++, but likewise is not part of ISO C++.
Citing the GCC Manual: Chapter 6 - Extensions to the C Language Family:
...
These extensions are available in C and Objective-C. Most of them are also available in C++. ...
- Statement Exprs: Putting statements and declarations inside expressions.
Where the latter is explained in detail in GCC Manual: Section 6.1 - Statements and Declarations in Expressions:
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the absolute value of
foo ().
...
As for Clang, the Clang Language Extensions describes that Clang aims to support many GCC extensions: [emhpasis mine]:
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
edited Nov 24 '18 at 11:46
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
answered Nov 24 '18 at 11:26
dfridfri
35.1k45696
35.1k45696
add a comment |
add a comment |
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2
This is not C++. If you want to be a language lawyer, you need to be
-pedantic.– n.m.
Nov 24 '18 at 11:11
I added the language-lawyer tag because of the mention of finding the relevant C++ standard paragraph. I should have checked that it actually compiled first. My bad. Removed it.
– Matthieu Brucher
Nov 24 '18 at 11:13
So whether I understand you right that a fact of successful compilation of this line with g++ 5.4 and clang++ 6.0 ("clang++-6.8 -std=c++14 -Wall -o 1.out 1.cpp") is an extension of both compilers? Upd: checked with -pedantic. Yes, a compiler says that is an extension.
– Vyacheslav Grigoryev
Nov 24 '18 at 11:14
A standard conforming way to do something like this would be to use a directly invoked lambda - like
int a = { int b = 10; return b; }();.– Jesper Juhl
Nov 24 '18 at 11:34