"explicit" 的使用

今天在编译项目时，代码审查提示 “Single-parameter constructors should be marked explicit”

于是就在构造函数前加上 explicit

下面为最小的示例，这个示例主要是按文件创建时间排序并删除过期的文件

#include <iostream>
#include <thread>
#include <windows.h>
#include <exception>
#include <vector>
#include <filesystem>

wchar_t file_path[] =
    L"D:/VS2019_Program/Test_SetUnhandledExceptionFilter/Debug/.sentry-native/reports/";

wchar_t file_ext[] = L".dmp";

std::vector<std::wstring> TraverseFolder(std::wstring folder_path,
                                         std::wstring ext);

class file {
 public:
  std::wstring name;
  FILETIME time;

 public:
  bool operator<(file const& other) const {
    return CompareFileTime(&other.time, &time) == 1;
  }

  file(WIN32_FIND_DATA const &d) : name(d.cFileName), time(d.ftCreationTime) {}
};

std::vector<file> SortDumpFiles(std::vector<std::wstring> dumpfiles) {
  std::vector<file> files;

  std::transform(dumpfiles.begin(), dumpfiles.end(), std::back_inserter(files),
                 [](std::wstring const &fname) {
                   WIN32_FIND_DATA d;
                   HANDLE h = FindFirstFile(fname.c_str(), &d);
                   FindClose(h);
                   return d;
                 });

  std::sort(files.begin(), files.end());

  return files;
}

void DelExpiredDumpFiles() {
  std::wstring path(file_path);
  std::wstring ext(file_ext);
  auto dump_file = TraverseFolder(path, ext);

  int total_file = dump_file.size();
  int to_be_deleted = total_file - 4;
  if (to_be_deleted <= 0) {
    return;
  }
  // 以文件创建时间
  auto files = SortDumpFiles(dump_file);
  while (to_be_deleted--) {
    std::wstring file = file_path;
    file += files[to_be_deleted].name;
    if (!DeleteFile(file.c_str())) {
      int err = GetLastError();
      return;
    }
  }
}

std::vector<std::wstring> TraverseFolder(std::wstring folder_path,
                                         std::wstring ext) {
  std::vector<std::wstring> filename;
  if (!std::filesystem::is_directory(folder_path)) {
    return filename;
  }

  for (auto& p : std::filesystem::recursive_directory_iterator(folder_path)) {
    if (p.path().extension() == ext) {
      filename.push_back(p.path().wstring());
    }
  }
  return filename;
}

int main() {
  DelExpiredDumpFiles();

  return 0;
}

添加 explicit 关键词

explicit file(WIN32_FIND_DATA const &d) : name(d.cFileName), time(d.ftCreationTime) {}

编译会报错，

解决方法，

  std::transform(dumpfiles.begin(), dumpfiles.end(), std::back_inserter(files),
                 [](std::wstring const &fname) {
                   WIN32_FIND_DATA d;
                   HANDLE h = FindFirstFile(fname.c_str(), &d);
                   FindClose(h);
                   return file{d};
                 });

原因：

explicit 关键字

C++ 参考手册如下解释

1. 指定构造函数或转换函数 (C++11起)为显式, 即它不能用于隐式转换和复制初始化.
2. explicit 指定符可以与常量表达式一同使用. 函数若且唯若该常量表达式求值为 true 才为显式. (C++20起)

也就是构造函数被 explicit 修饰后, 就不能再被隐式调用了

等于说，std::vector<file> files = d 这样的操作禁止使用了，只能 std::vector<file> files = files{d}

Effective C++中也阐述了:

被声明为 explicit的构造函数通常比其 non-explicit 兄弟更受欢迎, 因为它们禁止编译器执行非预期 (往往也不被期望) 的类型转换. 除非我有一个好理由允许构造函数被用于隐式类型转换, 否则我会把它声明为 explicit. 我鼓励你遵循相同的政策.