当使用/解析模板时停止编译 [英] Stop compilation when a template has been used/resolved
问题描述
这个MCVE:
#include
#include< time.h>
#define MAX_LENGTH_DATETIME 25
模板< typename T>
char * convertUnixTimeToChar(time_t unixTime,T)
{
(void)unixTime;
#pragma messageconvertUnixTimeToChar()中的第二个参数必须为< char [MAX_LENGTH_DATETIME]> ;.
exit(1);
};
模板< size_t N>
char * convertUnixTimeToChar(time_t unixTime,char(& destination)[N])
{
if(N {
printf(Overflow在convertUnixTimeToChar()中:destination size [%ld]必须至少为[%u],N,MAX_LENGTH_DATETIME);
exit(1);
}
struct tm * tmNow;
tmNow = localtime(& unixTime);
strftime(目的地,MAX_LENGTH_DATETIME - 1,%Y-%m-%d%H:%M:%S,tmNow);
返回目的地;
};
int main()
{
char buffer [MAX_LENGTH_DATETIME];
printf(Converted unix time =%s\\\
,convertUnixTimeToChar(1487585045,buffer));
}
我的问题:
我想要显示消息 #pragma messageconvertUnixTimeToChar()中的第二个参数必须是< char [MAX_LENGTH_DATETIME]>类型。$只有当 类型的
类型的变量传递给<$>时, c $ c> convertUnixTimeToChar()因为第一个模板已解析。现在我总是得到这个消息。
换句话说,如果传递错误的参数而不是程序运行时编译失败[在这种情况下,我必须使用 printf
而不是 #pragma
以获得通知]。
传递像 char [25]
这样的东西可以解决第二个模板,一切都很好!
有没有方式我可以有条件地停止编译错误消息,以防万一模板已被用于代码生成 ?
使用gcc编译4.9 .4使用这些开关: -Wall -Werror -Wextra -std = c ++ 11 -O3
再次,最佳解决方案是根本不做任何事情。
完全删除catch-all超载会导致编译器产生任何参数的错误那不能匹配 char(& destination)[N]
。
如果你想变得更加奇特,流n错误消息,你可以在catch-all中使用依赖的 static_assert
:
template< class ...,class T>
constexpr T&& depend(T&& o){
return std :: forward< T>(o);
}
模板< typename T>
char * convertUnixTimeToChar(time_t,T&&);
{
static_assert(取决于< T>(false),convertUnixTimeToChar()中的第二个参数必须...);
}
不是这可能不是更好的解决方案,因为你抓取了 convertUnixTimeToChar
的整个重载集合,并将其陷入一个硬错误,SFINAE会被诅咒。如果有人想为他们自己的类型添加重载,他们将不得不确保他们比这种爆炸式的捕获更好的匹配,这比收益更痛苦。
This MCVE:
#include <stdio.h>
#include <time.h>
#define MAX_LENGTH_DATETIME 25
template <typename T>
char * convertUnixTimeToChar( time_t unixTime, T )
{
(void) unixTime;
#pragma message "2nd parameter in convertUnixTimeToChar() must be of type <char [MAX_LENGTH_DATETIME]>."
exit(1);
};
template<size_t N>
char * convertUnixTimeToChar( time_t unixTime, char (&destination) [N] )
{
if ( N < MAX_LENGTH_DATETIME )
{
printf( "Overflow in convertUnixTimeToChar(): destination size [%ld] must be at least [%u]", N, MAX_LENGTH_DATETIME );
exit(1);
}
struct tm * tmNow;
tmNow = localtime( &unixTime );
strftime( destination, MAX_LENGTH_DATETIME - 1, "%Y-%m-%d %H:%M:%S", tmNow );
return destination;
};
int main()
{
char buffer [MAX_LENGTH_DATETIME ];
printf( "Converted unix time=%s\n", convertUnixTimeToChar( 1487585045, buffer ) );
}
My question:
I would like to get displayed the message #pragma message "2nd parameter in convertUnixTimeToChar() must be of type <char [MAX_LENGTH_DATETIME]>."
from gcc while the compilation only when a variable of type char *
has been passed to convertUnixTimeToChar()
because the 1st template was resolved. Now I get always this message.
In other words, the compilation should fail if a wrong parameter is passed and not when the program is running [in this case I have to use a printf
instead of #pragma
to get notified].
Passing something like char [25]
resolves the 2nd template and all is fine!
Is there a way I can stop conditionally the compilation with an error message in case a template has been used physically for the code generation?
Compiled with gcc 4.9.4 using these switches: -Wall -Werror -Wextra -std=c++11 -O3
Once more, the optimal solution is "don't do anything at all".
Removing the catch-all overload altogether will make the compiler produce an error for any parameter that cannot match char (&destination) [N]
.
If you want to get fancy and add your own error message, you can use a dependent static_assert
inside the catch-all:
template <class..., class T>
constexpr T &&depend(T &&o) {
return std::forward<T>(o);
}
template <typename T>
char *convertUnixTimeToChar( time_t, T&& )
{
static_assert(depend<T>(false), "2nd parameter in convertUnixTimeToChar() must ...");
}
Not that this is probably not a better solution, because you grab the whole overload set for convertUnixTimeToChar
and sink it into a hard error, SFINAE be damned. If someone want to add overloads for their own types, they will have to make sure theirs are a better match than this explosive catch-all, which is more pain than gain.
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