英文:
Resolve ambiguity
问题
我正在开发一个具有可变参数的方法，该方法起始于一个模板。
template<typename levelType, typename formatType, typename ... variadicParamsType>
void ClogLogger::log(levelType level, formatType format, variadicParamsType... formatParams)
{
string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << endl;
}
现在出现了错误：
log(0, "{}", "foo_1") // 打印 foo_1
log(0, "{} {}", "foo_1", "foo_2") // 错误，得到 <int, const char * e, const char * format, const char * formatParams>
另一方面，我有相同的方法，但还接受一个异常：
template<typename levelType, class exceptionType, typename formatType, typename ... variadicParamsType>
void log(levelType level, exceptionType e, formatType format, variadicParamsType ... formatParams)
{
string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << endl;
THROW_MYEXCEPTION(e.what());// 一个自定义宏
}
并且正常工作：
exception e;
log(0, e, "{}", "foo_1", "foo_2") // 现在正常工作
更新：
我似乎找到了问题，模板展开在以下情况下产生了歧义：
log(0, "{}{}" "foo_1", "foo_2")
这会展开为：
<int level, const char * e, const char * format, const char * formatParams>
这在调用 e.what() 时会导致错误。
您知道如何强制编译器使用第一个方法展开吗？即不包含异常的方法。
英文:
I am developing a method with variatic arguments that starts from a template.
template<typename levelType, typename formatType, typename ... variadicParamsType>
void ClogLogger::log(levelType level, formatType format, variadicParamsType... formatParams)
{
string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << endl;
}
Now the error:
log(0, "{}", "foo_1") // Prints foo_1
log(0, "{} {}", "foo_1", "foo_2") // Error, getting <int, const char * e, const char * format, const char * formatParams>
On the other hand I have this same method, but accepting also an exception:
template<typename levelType, class exceptionType, typename formatType, typename ... variadicParamsType>
void log(levelType level, exceptionType e, formatType format, variadicParamsType ... formatParams)
{
string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << endl;
THROW_MYEXCEPTION(e.what());// A custom macro
}
And performing well:
exception e;
log(0, e, {}{}, "foo_1", "foo_2") // Now, work´s fine
UPDATE:
I seem to have found the problem, the template expansion produces an ambiguity in the case:
log(0, "{}{}" "foo_1", "foo_2")
That expands as:
<int level, const char * e, const char * format, const char * formatParams>
This causes an error when e.what() is launched.
Do you know how I can force the compiler to expand using the first method? The one that does not contain the exception.
答案1
得分: 1
以下是翻译好的部分：
大致如下，也许：
template<typename levelType, class exceptionType, typename formatType,
typename ... variadicParamsType>
auto log(levelType level, const exceptionType& e, formatType format,
variadicParamsType... formatParams)
-> std::enable_if_t<(sizeof(decltype(e.what())*)>0)>
{
std::string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << std::endl;
THROW_MYEXCEPTION(e.what());// A custom macro
}
[演示](https://gcc.godbolt.org/z/3MW1sKhev)
这个想法是在重载决议中排除第二个重载，除非`e`“看起来像一个异常”，定义为“`e.what()`是一个良好形式的表达式”（因为函数的主体依赖于`e.what()`可用）。
请注意，代码中的注释和链接未翻译。如果需要更多帮助，请告诉我。
英文:
Something along these lines, perhaps:
template<typename levelType, class exceptionType, typename formatType,
typename ... variadicParamsType>
auto log(levelType level, const exceptionType& e, formatType format,
variadicParamsType... formatParams)
-> std::enable_if_t<(sizeof(decltype(e.what())*)>0)>
{
std::string levelHeader = getLevelHeader(level);
clog << levelHeader << fmt::format(format, formatParams...) << std::endl;
THROW_MYEXCEPTION(e.what());// A custom macro
}
The idea is to exclude the second overload from overload resolution unless e "looks like an exception", defined as "e.what() is a well-formed expression" (seeing as the body of the function relies on e.what() being available).
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