Why is boost::optional not convertable to bool for the purposes of std::is_convertible
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4
I have
auto result = std::is_convertible
< boost::optional<int>
, bool
>::value;
static_assert( result , "task should return bool" );
and it fails to compile. The definition of std::is_convertible is
template< class From, class To > struct is_convertible;
and optional is clearly convertible to boolean because we always use it like
void(boost::optional<int> const & value)
if(value)
std::cerr << *value << endl;
what am I missing here?
c++ boost-optional
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
add a comment |
4
I have
auto result = std::is_convertible
< boost::optional<int>
, bool
>::value;
static_assert( result , "task should return bool" );
and it fails to compile. The definition of std::is_convertible is
template< class From, class To > struct is_convertible;
and optional is clearly convertible to boolean because we always use it like
void(boost::optional<int> const & value)
if(value)
std::cerr << *value << endl;
what am I missing here?
c++ boost-optional
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
add a comment |
4
4
4
I have
auto result = std::is_convertible
< boost::optional<int>
, bool
>::value;
static_assert( result , "task should return bool" );
and it fails to compile. The definition of std::is_convertible is
template< class From, class To > struct is_convertible;
and optional is clearly convertible to boolean because we always use it like
void(boost::optional<int> const & value)
if(value)
std::cerr << *value << endl;
what am I missing here?
c++ boost-optional
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
I have
auto result = std::is_convertible
< boost::optional<int>
, bool
>::value;
static_assert( result , "task should return bool" );
and it fails to compile. The definition of std::is_convertible is
template< class From, class To > struct is_convertible;
and optional is clearly convertible to boolean because we always use it like
void(boost::optional<int> const & value)
if(value)
std::cerr << *value << endl;
what am I missing here?
c++ boost-optional
c++ boost-optional
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
asked Nov 15 '18 at 12:11
bradgonesurfingbradgonesurfing
16.6k1084154
16.6k1084154
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
10
boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.
You need std::is_constructible, which tries to perform an explicit conversion.
The following compiles
static_assert
( std::is_constructible<bool, boost::optional<int>>::value
, "msg" );
and the following fails to compile because optional is not convertible to int
static_assert
( std::is_constructible<int, boost::optional<int>>::value
, "msg" );
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared tostd::is_convertible, but it should be as simple asstd::is_constructible<bool, boost::optional<int>>.
– Quentin
Nov 15 '18 at 12:26
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
10
boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.
You need std::is_constructible, which tries to perform an explicit conversion.
The following compiles
static_assert
( std::is_constructible<bool, boost::optional<int>>::value
, "msg" );
and the following fails to compile because optional is not convertible to int
static_assert
( std::is_constructible<int, boost::optional<int>>::value
, "msg" );
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared tostd::is_convertible, but it should be as simple asstd::is_constructible<bool, boost::optional<int>>.
– Quentin
Nov 15 '18 at 12:26
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
add a comment |
10
boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.
You need std::is_constructible, which tries to perform an explicit conversion.
The following compiles
static_assert
( std::is_constructible<bool, boost::optional<int>>::value
, "msg" );
and the following fails to compile because optional is not convertible to int
static_assert
( std::is_constructible<int, boost::optional<int>>::value
, "msg" );
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared tostd::is_convertible, but it should be as simple asstd::is_constructible<bool, boost::optional<int>>.
– Quentin
Nov 15 '18 at 12:26
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
add a comment |
10
10
10
boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.
You need std::is_constructible, which tries to perform an explicit conversion.
The following compiles
static_assert
( std::is_constructible<bool, boost::optional<int>>::value
, "msg" );
and the following fails to compile because optional is not convertible to int
static_assert
( std::is_constructible<int, boost::optional<int>>::value
, "msg" );
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.
You need std::is_constructible, which tries to perform an explicit conversion.
The following compiles
static_assert
( std::is_constructible<bool, boost::optional<int>>::value
, "msg" );
and the following fails to compile because optional is not convertible to int
static_assert
( std::is_constructible<int, boost::optional<int>>::value
, "msg" );
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
edited Nov 15 '18 at 12:31
bradgonesurfing
16.6k1084154
16.6k1084154
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
answered Nov 15 '18 at 12:19
QuentinQuentin
47.1k692148
47.1k692148
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared tostd::is_convertible, but it should be as simple asstd::is_constructible<bool, boost::optional<int>>.
– Quentin
Nov 15 '18 at 12:26
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
add a comment |
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared tostd::is_convertible, but it should be as simple asstd::is_constructible<bool, boost::optional<int>>.
– Quentin
Nov 15 '18 at 12:26
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.
– bradgonesurfing
Nov 15 '18 at 12:25
@bradgonesurfing There's a gotcha in that the arguments are reversed compared to
std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.– Quentin
Nov 15 '18 at 12:26
@bradgonesurfing There's a gotcha in that the arguments are reversed compared to
std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.– Quentin
Nov 15 '18 at 12:26
1
1
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)
– bradgonesurfing
Nov 15 '18 at 12:32
add a comment |
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