Templates and ODR
The question is about the C++ documentation and standard documentation. Is in the following code the variable x odr-used?
extern int x;
template<class T> T f() { return x; }
It seems to me it is not used, bud where it is stated in documents? (there is the statement, that appearance x in expression is odr-use, but...)
c++ templates language-lawyer
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
add a comment |
The question is about the C++ documentation and standard documentation. Is in the following code the variable x odr-used?
extern int x;
template<class T> T f() { return x; }
It seems to me it is not used, bud where it is stated in documents? (there is the statement, that appearance x in expression is odr-use, but...)
c++ templates language-lawyer
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
add a comment |
The question is about the C++ documentation and standard documentation. Is in the following code the variable x odr-used?
extern int x;
template<class T> T f() { return x; }
It seems to me it is not used, bud where it is stated in documents? (there is the statement, that appearance x in expression is odr-use, but...)
c++ templates language-lawyer
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
The question is about the C++ documentation and standard documentation. Is in the following code the variable x odr-used?
extern int x;
template<class T> T f() { return x; }
It seems to me it is not used, bud where it is stated in documents? (there is the statement, that appearance x in expression is odr-use, but...)
c++ templates language-lawyer
c++ templates language-lawyer
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
edited Nov 20 '18 at 8:00
StoryTeller
93.7k12187253
93.7k12187253
asked Nov 20 '18 at 7:51
guest
542
asked Nov 20 '18 at 7:51
guest
542
asked Nov 20 '18 at 7:51
guest
542
542
add a comment |
add a comment |
1 Answer
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It is odr-used. [basic.def.odr]/2:
An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.
Thus the expression x is potentially evaluated.
[basic.def.odr]/4:
A variable
xwhose name appears as a potentially-evaluated expressionexis odr-used byexunless applying the lvalue-to-rvalue conversion ([conv.lval]) toxyields a constant expression ([expr.const]) [...]
We can stop here. Applying the lvalue-to-rvalue conversion to x does not yield a constant expression.
This violation of the ODR does not require a diagnostic ([basic.def.odr]/10):
Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program outside of a
discarded statement; no diagnostic required.
Even assuming for the sake of argument that f does not odr-use x until/unless a specialization is generated, it still won't help the program, because the program is also ill-formed NDR if "no valid specialization can be generated for a template" ([temp.res]/8), and since every valid specialization of f undoubtedly odr-uses x, no valid specialization can be generated if a definition of x is not present in the program.
answered Nov 20 '18 at 8:50
T.C.
106k13216321
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
add a comment |
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1 Answer
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1 Answer
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active
oldest
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active
oldest
votes
It is odr-used. [basic.def.odr]/2:
An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.
Thus the expression x is potentially evaluated.
[basic.def.odr]/4:
A variable
xwhose name appears as a potentially-evaluated expressionexis odr-used byexunless applying the lvalue-to-rvalue conversion ([conv.lval]) toxyields a constant expression ([expr.const]) [...]
We can stop here. Applying the lvalue-to-rvalue conversion to x does not yield a constant expression.
This violation of the ODR does not require a diagnostic ([basic.def.odr]/10):
Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program outside of a
discarded statement; no diagnostic required.
Even assuming for the sake of argument that f does not odr-use x until/unless a specialization is generated, it still won't help the program, because the program is also ill-formed NDR if "no valid specialization can be generated for a template" ([temp.res]/8), and since every valid specialization of f undoubtedly odr-uses x, no valid specialization can be generated if a definition of x is not present in the program.
answered Nov 20 '18 at 8:50
T.C.
106k13216321
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
add a comment |
It is odr-used. [basic.def.odr]/2:
An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.
Thus the expression x is potentially evaluated.
[basic.def.odr]/4:
A variable
xwhose name appears as a potentially-evaluated expressionexis odr-used byexunless applying the lvalue-to-rvalue conversion ([conv.lval]) toxyields a constant expression ([expr.const]) [...]
We can stop here. Applying the lvalue-to-rvalue conversion to x does not yield a constant expression.
This violation of the ODR does not require a diagnostic ([basic.def.odr]/10):
Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program outside of a
discarded statement; no diagnostic required.
Even assuming for the sake of argument that f does not odr-use x until/unless a specialization is generated, it still won't help the program, because the program is also ill-formed NDR if "no valid specialization can be generated for a template" ([temp.res]/8), and since every valid specialization of f undoubtedly odr-uses x, no valid specialization can be generated if a definition of x is not present in the program.
answered Nov 20 '18 at 8:50
T.C.
106k13216321
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
add a comment |
It is odr-used. [basic.def.odr]/2:
An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.
Thus the expression x is potentially evaluated.
[basic.def.odr]/4:
A variable
xwhose name appears as a potentially-evaluated expressionexis odr-used byexunless applying the lvalue-to-rvalue conversion ([conv.lval]) toxyields a constant expression ([expr.const]) [...]
We can stop here. Applying the lvalue-to-rvalue conversion to x does not yield a constant expression.
This violation of the ODR does not require a diagnostic ([basic.def.odr]/10):
Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program outside of a
discarded statement; no diagnostic required.
Even assuming for the sake of argument that f does not odr-use x until/unless a specialization is generated, it still won't help the program, because the program is also ill-formed NDR if "no valid specialization can be generated for a template" ([temp.res]/8), and since every valid specialization of f undoubtedly odr-uses x, no valid specialization can be generated if a definition of x is not present in the program.
answered Nov 20 '18 at 8:50
T.C.
106k13216321
It is odr-used. [basic.def.odr]/2:
An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.
Thus the expression x is potentially evaluated.
[basic.def.odr]/4:
A variable
xwhose name appears as a potentially-evaluated expressionexis odr-used byexunless applying the lvalue-to-rvalue conversion ([conv.lval]) toxyields a constant expression ([expr.const]) [...]
We can stop here. Applying the lvalue-to-rvalue conversion to x does not yield a constant expression.
This violation of the ODR does not require a diagnostic ([basic.def.odr]/10):
Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program outside of a
discarded statement; no diagnostic required.
Even assuming for the sake of argument that f does not odr-use x until/unless a specialization is generated, it still won't help the program, because the program is also ill-formed NDR if "no valid specialization can be generated for a template" ([temp.res]/8), and since every valid specialization of f undoubtedly odr-uses x, no valid specialization can be generated if a definition of x is not present in the program.
answered Nov 20 '18 at 8:50
T.C.
106k13216321
answered Nov 20 '18 at 8:50
T.C.
106k13216321
answered Nov 20 '18 at 8:50
T.C.
106k13216321
answered Nov 20 '18 at 8:50
T.C.
106k13216321
106k13216321
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
add a comment |
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
What then about instantiation of templates when they are used? If x were a template or static data member of a class template... It should be then instantiated. But I don't found compilers to do this...
– guest
Nov 20 '18 at 9:37
add a comment |
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