diff options
Diffstat (limited to 'absl/flags/internal/flag.h')
| -rw-r--r-- | absl/flags/internal/flag.h | 89 |
1 files changed, 79 insertions, 10 deletions
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h index ce0ccf2d31d2..d79902543a22 100644 --- a/absl/flags/internal/flag.h +++ b/absl/flags/internal/flag.h @@ -61,6 +61,68 @@ class FlagState : public flags_internal::FlagStateInterface { int64_t counter_; }; +// This is help argument for absl::Flag encapsulating the string literal pointer +// or pointer to function generating it as well as enum descriminating two +// cases. +using HelpGenFunc = std::string (*)(); + +union FlagHelpSrc { + constexpr explicit FlagHelpSrc(const char* help_msg) : literal(help_msg) {} + constexpr explicit FlagHelpSrc(HelpGenFunc help_gen) : gen_func(help_gen) {} + + const char* literal; + HelpGenFunc gen_func; +}; + +enum class FlagHelpSrcKind : int8_t { kLiteral, kGenFunc }; + +struct HelpInitArg { + FlagHelpSrc source; + FlagHelpSrcKind kind; +}; + +// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by +// ABSL_FLAG macro. It is only used to silence the compiler in the case where +// help message expression is not constexpr and does not have type const char*. +// If help message expression is indeed constexpr const char* HelpConstexprWrap +// is just a trivial identity function. +template <typename T> +const char* HelpConstexprWrap(const T&) { + return nullptr; +} +constexpr const char* HelpConstexprWrap(const char* p) { return p; } +constexpr const char* HelpConstexprWrap(char* p) { return p; } + +// These two HelpArg overloads allows us to select at compile time one of two +// way to pass Help argument to absl::Flag. We'll be passing +// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer +// first overload if possible. If T::Const is evaluatable on constexpr +// context (see non template int parameter below) we'll choose first overload. +// In this case the help message expression is immediately evaluated and is used +// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG. +// Otherwise SFINAE kicks in and first overload is dropped from the +// consideration, in which case the second overload will be used. The second +// overload does not attempt to evaluate the help message expression +// immediately and instead delays the evaluation by returing the function +// pointer (&T::NonConst) genering the help message when necessary. This is +// evaluatable in constexpr context, but the cost is an extra function being +// generated in the ABSL_FLAG code. +template <typename T, int = (T::Const(), 1)> +constexpr flags_internal::HelpInitArg HelpArg(int) { + return {flags_internal::FlagHelpSrc(T::Const()), + flags_internal::FlagHelpSrcKind::kLiteral}; +} + +template <typename T> +constexpr flags_internal::HelpInitArg HelpArg(char) { + return {flags_internal::FlagHelpSrc(&T::NonConst), + flags_internal::FlagHelpSrcKind::kGenFunc}; +} + +// Signature for the function generating the initial flag value based (usually +// based on default value supplied in flag's definition) +using InitialValGenFunc = void* (*)(); + // Signature for the mutation callback used by watched Flags // The callback is noexcept. // TODO(rogeeff): add noexcept after C++17 support is added. @@ -74,15 +136,19 @@ class FlagImpl { public: constexpr FlagImpl(const flags_internal::FlagOpFn op, const flags_internal::FlagMarshallingOpFn marshalling_op, - const flags_internal::InitialValGenFunc initial_value_gen) + const flags_internal::InitialValGenFunc initial_value_gen, + const HelpInitArg help) : op_(op), marshalling_op_(marshalling_op), - initial_value_gen_(initial_value_gen) {} + initial_value_gen_(initial_value_gen), + help_(help.source), + help_source_kind_(help.kind) {} // Forces destruction of the Flag's data. void Destroy() const; // Constant access methods + std::string Help() const; bool IsModified() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu); bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu); std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu); @@ -149,9 +215,12 @@ class FlagImpl { absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED(locks_->primary_mu); // Immutable Flag's data. - const FlagOpFn op_; // Type-specific handler - const FlagMarshallingOpFn marshalling_op_; // Marshalling ops handler - const InitialValGenFunc initial_value_gen_; // Makes flag's initial value + const FlagOpFn op_; // Type-specific handler. + const FlagMarshallingOpFn marshalling_op_; // Marshalling ops handler. + const InitialValGenFunc initial_value_gen_; // Makes flag's initial value. + const FlagHelpSrc help_; // Help message literal or function to generate it. + // Indicates if help message was supplied as literal or generator func. + const FlagHelpSrcKind help_source_kind_; // Mutable Flag's data. (guarded by locks_->primary_mu). // Indicates that locks_, cur_ and def_ fields have been lazily initialized. @@ -191,14 +260,13 @@ class FlagImpl { template <typename T> class Flag final : public flags_internal::CommandLineFlag { public: - constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen, + constexpr Flag(const char* name, const flags_internal::HelpInitArg help, const char* filename, const flags_internal::FlagMarshallingOpFn marshalling_op, const flags_internal::InitialValGenFunc initial_value_gen) - : flags_internal::CommandLineFlag( - name, flags_internal::HelpText::FromFunctionPointer(help_gen), - filename), - impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen) {} + : flags_internal::CommandLineFlag(name, filename), + impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen, + help) {} T Get() const { // See implementation notes in CommandLineFlag::Get(). @@ -222,6 +290,7 @@ class Flag final : public flags_internal::CommandLineFlag { } // CommandLineFlag interface + std::string Help() const override { return impl_.Help(); } bool IsModified() const override { return impl_.IsModified(); } bool IsSpecifiedOnCommandLine() const override { return impl_.IsSpecifiedOnCommandLine(); |