compile-time-constant

问题 I am making an expression parser for a calculator. The expressions will contain a variable, for instance, a user could enter "x + 2", or "y^2". I have a switch statement, and one of the cases in switch statement performs a certain action when it detects a variable: case variableSymbol: if (expression.length() == 1) { rangeResult = x1; break outer; } varFlag = true; varPos = expresPos; break; Originally, I hard coded a value 'x' in the above case, but I would like to give users a choice as to

问题 I am making an expression parser for a calculator. The expressions will contain a variable, for instance, a user could enter "x + 2", or "y^2". I have a switch statement, and one of the cases in switch statement performs a certain action when it detects a variable: case variableSymbol: if (expression.length() == 1) { rangeResult = x1; break outer; } varFlag = true; varPos = expresPos; break; Originally, I hard coded a value 'x' in the above case, but I would like to give users a choice as to

问题 I am making an expression parser for a calculator. The expressions will contain a variable, for instance, a user could enter "x + 2", or "y^2". I have a switch statement, and one of the cases in switch statement performs a certain action when it detects a variable: case variableSymbol: if (expression.length() == 1) { rangeResult = x1; break outer; } varFlag = true; varPos = expresPos; break; Originally, I hard coded a value 'x' in the above case, but I would like to give users a choice as to

问题 I'm trying to use Boost MPL and Fusion to calculate the size of a struct exclusive of any padding. This is my current best attempt: Live example template<class T> constexpr std::size_t sizeof_members(void) { using namespace std; namespace mpl = boost::mpl; namespace fusion = boost::fusion; //This works, but only for structs containing exactly 4 members... typedef typename mpl::apply<mpl::unpack_args<mpl::vector<mpl::_1, mpl::_2, mpl::_3, mpl::_4>::type >, T>::type member_types; typedef

问题 I'm trying to use Boost MPL and Fusion to calculate the size of a struct exclusive of any padding. This is my current best attempt: Live example template<class T> constexpr std::size_t sizeof_members(void) { using namespace std; namespace mpl = boost::mpl; namespace fusion = boost::fusion; //This works, but only for structs containing exactly 4 members... typedef typename mpl::apply<mpl::unpack_args<mpl::vector<mpl::_1, mpl::_2, mpl::_3, mpl::_4>::type >, T>::type member_types; typedef

问题 I understood what a compile time constant rule is from Compile-time constants and variables. declared as final have a primitive or String type initialized at the same time as the declaration initialized with constant expression final int x = 5; But I fail to understand why below code doesn't: final int x; x = 5; The only difference is of third point above. How initialization on different line instead of same line makes a difference. 回答1: Case 1 final int x = 5; public static void main(String[

问题 I understood what a compile time constant rule is from Compile-time constants and variables. declared as final have a primitive or String type initialized at the same time as the declaration initialized with constant expression final int x = 5; But I fail to understand why below code doesn't: final int x; x = 5; The only difference is of third point above. How initialization on different line instead of same line makes a difference. 回答1: Case 1 final int x = 5; public static void main(String[

问题 I have a requirement to build an automated system to parse a C++ .h file with a lot of #define statements in it and do something with the value that each #define works out to. The .h file has a lot of other junk in it besides the #define statements. The objective is to create a key-value list, where the keys are all the keywords defined by the #define statements and the values are the evaluations of the macros which correspond to the definitions. The #defines define the keywords with a series

问题 I have a requirement to build an automated system to parse a C++ .h file with a lot of #define statements in it and do something with the value that each #define works out to. The .h file has a lot of other junk in it besides the #define statements. The objective is to create a key-value list, where the keys are all the keywords defined by the #define statements and the values are the evaluations of the macros which correspond to the definitions. The #defines define the keywords with a series

问题 Is it possible to declare a new type (an empty struct , or a struct without an implementation) on the fly? E.g. constexpr auto make_new_type() -> ???; using A = decltype(make_new_type()); using B = decltype(make_new_type()); using C = decltype(make_new_type()); static_assert(!std::is_same<A, B>::value, ""); static_assert(!std::is_same<B, C>::value, ""); static_assert(!std::is_same<A, C>::value, ""); A "manual" solution is template <class> struct Tag; using A = Tag<struct TagA>; using B = Tag