StringSwitch.h

Go to the documentation of this file.

//===--- StringSwitch.h - Switch-on-literal-string Construct --------------===/
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//===----------------------------------------------------------------------===/
///
/// \file
///  This file implements the StringSwitch template, which mimics a switch()
///  statement whose cases are string literals.
///
//===----------------------------------------------------------------------===/
#ifndef LLVM_ADT_STRINGSWITCH_H
#define LLVM_ADT_STRINGSWITCH_H
 
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/ErrorHandling.h"
#include <cassert>
#include <cstring>
#include <initializer_list>
#include <optional>
 
namespace llvm {
 
/// A switch()-like statement whose cases are string literals.
///
/// The StringSwitch class is a simple form of a switch() statement that
/// determines whether the given string matches one of the given string
/// literals. The template type parameter \p T is the type of the value that
/// will be returned from the string-switch expression. For example,
/// the following code switches on the name of a color in \c argv[i]:
///
/// \code
/// Color color = StringSwitch<Color>(argv[i])
///   .Case("red", Red)
///   .Case("orange", Orange)
///   .Case("yellow", Yellow)
///   .Case("green", Green)
///   .Case("blue", Blue)
///   .Case("indigo", Indigo)
///   .Cases({"violet", "purple"}, Violet)
///   .Default(UnknownColor);
/// \endcode
///
/// When multiple matches are found, the value of the first match is returned.
template<typename T, typename R = T>
class StringSwitch {
  /// The string we are matching.
  const StringRef Str;
 
  /// The pointer to the result of this switch statement, once known,
  /// null before that.
  std::optional<T> Result;
 
public:
  explicit StringSwitch(StringRef S)
  : Str(S), Result() { }
 
  StringSwitch(StringSwitch &&) = default;
 
  // StringSwitch is not copyable.
  StringSwitch(const StringSwitch &) = delete;
 
  // StringSwitch is not assignable due to 'Str' being 'const'.
  void operator=(const StringSwitch &) = delete;
  void operator=(StringSwitch &&) = delete;
 
  // Case-sensitive case matchers.
  StringSwitch &Case(StringLiteral S, T Value) {
    CaseImpl(S, Value);
    return *this;
  }
 
  StringSwitch& EndsWith(StringLiteral S, T Value) {
    if (!Result && Str.ends_with(S)) {
      Result = std::move(Value);
    }
    return *this;
  }
 
  StringSwitch& StartsWith(StringLiteral S, T Value) {
    if (!Result && Str.starts_with(S)) {
      Result = std::move(Value);
    }
    return *this;
  }
 
  StringSwitch &Cases(std::initializer_list<StringLiteral> CaseStrings,
                      T Value) {
    // Stop matching after the string is found.
    for (StringLiteral S : CaseStrings)
      if (CaseImpl(S, Value))
        break;
    return *this;
  }
 
  // Case-insensitive case matchers.
  StringSwitch &CaseLower(StringLiteral S, T Value) {
    CaseLowerImpl(S, Value);
    return *this;
  }
 
  StringSwitch &EndsWithLower(StringLiteral S, T Value) {
    if (!Result && Str.ends_with_insensitive(S))
      Result = std::move(Value);
 
    return *this;
  }
 
  StringSwitch &StartsWithLower(StringLiteral S, T Value) {
    if (!Result && Str.starts_with_insensitive(S))
      Result = std::move(Value);
 
    return *this;
  }
 
  StringSwitch &CasesLower(std::initializer_list<StringLiteral> CaseStrings,
                           T Value) {
    // Stop matching after the string is found.
    for (StringLiteral S : CaseStrings)
      if (CaseLowerImpl(S, Value))
        break;
    return *this;
  }
 
  // A StringSwitch case that is selected if the specified predicate
  // returns true for the subject string.
  StringSwitch &Predicate(function_ref<bool(StringRef)> Pred, T Value) {
    if (!Result && Pred(Str))
      Result = std::move(Value);
    return *this;
  }
 
  [[nodiscard]] R Default(T Value) {
    if (Result)
      return std::move(*Result);
    return Value;
  }
 
  /// Declare default as unreachable, making sure that all cases were handled.
  [[nodiscard]] R DefaultUnreachable(
      const char *Message = "Fell off the end of a string-switch") {
    if (Result)
      return std::move(*Result);
    llvm_unreachable(Message);
  }
 
  [[nodiscard]] operator R() { return DefaultUnreachable(); }
 
private:
  // Returns true when a match is found. If `Str` matches the `S` argument,
  // stores the result.
  bool CaseImpl(StringLiteral S, T &Value) {
    if (Result)
      return true;
 
    if (Str != S)
      return false;
 
    Result = std::move(Value);
    return true;
  }
 
  // Returns true when a match is found. If `Str` matches the `S` argument
  // (case-insensitive), stores the result.
  bool CaseLowerImpl(StringLiteral S, T &Value) {
    if (Result)
      return true;
 
    if (!Str.equals_insensitive(S))
      return false;
 
    Result = std::move(Value);
    return true;
  }
};
 
} // end namespace llvm
 
#endif // LLVM_ADT_STRINGSWITCH_H