c++17

问题 I have a template class with 3 template arguments. template <class T, class U, class Y> class MyClass {}; I wanna get input from users by CLI arguments, something like ./cli float driver-x load The first arg can be float or double The second arg is a driver name: driver-x , driver-y , ... The third argument is about the action type: load , unload , ... If I want to create a new instance of MyClass based on user inputs, I have to define many if/else statements. Because a user inputs are string

问题 I'm going to start with how I imagine using the code I'd like to create. It doesn't have to be exactly like this but it's a good example of what I mean by "concise" in the title. In my case it's mapping of a type to a related enumeration value. struct bar : foo<bar, foo_type::bar> { /* ... */ }; // \_/ \___________/ // ^ Type ^ Value What this should ideally do is an automatic registration of a bidirectional mapping between the first template parameter of foo , a type, and second, a value,

问题 I'm going to start with how I imagine using the code I'd like to create. It doesn't have to be exactly like this but it's a good example of what I mean by "concise" in the title. In my case it's mapping of a type to a related enumeration value. struct bar : foo<bar, foo_type::bar> { /* ... */ }; // \_/ \___________/ // ^ Type ^ Value What this should ideally do is an automatic registration of a bidirectional mapping between the first template parameter of foo , a type, and second, a value,

问题 C++17 allows static member variables to be defined thus: class X { public: static inline int i = 8; }; What is the rationale behind requiring the inline specification? Why not simply allow programmers to write static int i = 8; in the class? 回答1: Without inline , it's explicitly stated as only a declaration. As specified in [class.static.data]/2 The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void.

问题 C++17 allows static member variables to be defined thus: class X { public: static inline int i = 8; }; What is the rationale behind requiring the inline specification? Why not simply allow programmers to write static int i = 8; in the class? 回答1: Without inline , it's explicitly stated as only a declaration. As specified in [class.static.data]/2 The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void.

问题 I was playing around with auto template parameters and I was surprised that this code didn't compiled: constexpr auto bar = 2; template<auto& T> struct Foo { auto operator()() const { return T; } }; int main() { Foo<bar> b; b(); } Visual Studio 15.7 (preview 4) spit out these errors: error C2970: 'Foo': template parameter 'T': 'bar': an expression involving objects with internal linkage cannot be used as a non-type argument note: see declaration of 'Foo' note: see declaration of 'bar' error

问题 I was playing around with auto template parameters and I was surprised that this code didn't compiled: constexpr auto bar = 2; template<auto& T> struct Foo { auto operator()() const { return T; } }; int main() { Foo<bar> b; b(); } Visual Studio 15.7 (preview 4) spit out these errors: error C2970: 'Foo': template parameter 'T': 'bar': an expression involving objects with internal linkage cannot be used as a non-type argument note: see declaration of 'Foo' note: see declaration of 'bar' error

问题 So I've been trying to include the <filesystem> into my project, which seem to be a bigger problem than I thought. <filesystem> should be part of c++17, I need to add that definition into my CMakeList. My root CmakeLists look like this: MESSAGE(“In src CMAKELIST”) # # Build everything in include/ directory add_subdirectory(include) # #set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib) #set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib) set(CMAKE_RUNTIME_OUTPUT_DIRECTORY $

问题 So I've been trying to include the <filesystem> into my project, which seem to be a bigger problem than I thought. <filesystem> should be part of c++17, I need to add that definition into my CMakeList. My root CmakeLists look like this: MESSAGE(“In src CMAKELIST”) # # Build everything in include/ directory add_subdirectory(include) # #set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib) #set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib) set(CMAKE_RUNTIME_OUTPUT_DIRECTORY $

问题 In the same translation unit, ODR problems are easy to diagnose. Why then does the compiler not warn against ODR violations in the same translation unit? For example in the following code https://wandbox.org/permlink/I0iyGdyw9ynRgny6 (reproduced below), there is an ODR violation with detecting if std::tuple_size has been defined. And further the undefined behavior is evident when you uncomment the defintiions of three and four . The output of the program changes. Just trying to understand why