C++中的typedef

语法 typedef

C++

发布日期: 2020-12-05

文章字数: 304

阅读时长: 1 分

typedef

几个用途:

定义类型的别名

typedef char* PCHAR

typedef + struct 定义结构体,其实与别名本质上是一样的

typedef struct {
    char* name;
    int age;
    int gender;
    char* address;
} Person;

// person是一个变量
Person person;

函数指针

一般来说,定义函数指针并不需要使用typedef,但是使用typedef会更加的直观:

#include <iostream>

typedef int (*ComputeSum)(int a, int b);

ComputeSum sum;

int sumFunc(int a, int b) {
   return a + b;
}

int main() {
    sum = sumFunc;
    int result = sum(1,2);
    std::cout << result << std::endl;
    return 0;
}

函数指针的格式: 返回类型 (*函数名字)(形式参数);

再举一个实际点的例子吧(Flutter Engine中的一段):

在embedder.h中有:

typedef FlutterEngineResult (*FlutterEngineOnVsyncFnPtr)(
    FLUTTER_API_SYMBOL(FlutterEngine) engine,
    intptr_t baton,
    uint64_t frame_start_time_nanos,
    uint64_t frame_target_time_nanos);

FlutterEngineOnVsyncFnPtr OnVsync;

在embedder.cc中有实现:

#define SET_PROC(member, function)        \
  if (STRUCT_HAS_MEMBER(table, member)) { \
    table->member = &function;            \
  }

SET_PROC(OnVsync, FlutterEngineOnVsync);

FlutterEngineResult FlutterEngineOnVsync(FLUTTER_API_SYMBOL(FlutterEngine)
                                             engine,
                                         intptr_t baton,
                                         uint64_t frame_start_time_nanos,
                                         uint64_t frame_target_time_nanos) {
  if (engine == nullptr) {
    return LOG_EMBEDDER_ERROR(kInvalidArguments, "Invalid engine handle.");
  }

  TRACE_EVENT0("flutter", "FlutterEngineOnVsync");

  auto start_time = fml::TimePoint::FromEpochDelta(
      fml::TimeDelta::FromNanoseconds(frame_start_time_nanos));

  auto target_time = fml::TimePoint::FromEpochDelta(
      fml::TimeDelta::FromNanoseconds(frame_target_time_nanos));

  if (!reinterpret_cast<flutter::EmbedderEngine*>(engine)->OnVsyncEvent(
          baton, start_time, target_time)) {
    return LOG_EMBEDDER_ERROR(
        kInternalInconsistency,
        "Could not notify the running engine instance of a Vsync event.");
  }

  return kSuccess;
}

FlutterEngineResult FlutterEngineReloadSystemFonts(
    FLUTTER_API_SYMBOL(FlutterEngine) engine) {
  if (engine == nullptr) {
    return LOG_EMBEDDER_ERROR(kInvalidArguments, "Invalid engine handle.");
  }

  TRACE_EVENT0("flutter", "FlutterEngineReloadSystemFonts");

  if (!reinterpret_cast<flutter::EmbedderEngine*>(engine)
           ->ReloadSystemFonts()) {
    return LOG_EMBEDDER_ERROR(kInternalInconsistency,
                              "Could not reload system fonts.");
  }

  return kSuccess;
}