1 // Copyright 2016 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef TOOLS_GN_PARSE_NODE_VALUE_ADAPTER_H_ 6 #define TOOLS_GN_PARSE_NODE_VALUE_ADAPTER_H_ 7 8 #include "gn/value.h" 9 10 class ParseNode; 11 12 // Provides a means to convert a parse node to a value without causing a copy 13 // in the common case of an "Identifier" node. Normally to get a value from a 14 // parse node you have to call Execute(), and when an identifier is executed 15 // it just returns the current value of itself as a copy. But some variables 16 // are very large (lists of many strings for example). 17 // 18 // The reason you might not want to do this is that in the case of an 19 // identifier where the copy is optimized away, the origin will still be the 20 // original value. The result can be confusing because it will reference the 21 // original value rather than the place where the value was dereferenced, e.g. 22 // for a function call. The InitForType() function will verify type information 23 // and will fix up the origin so it's not confusing. 24 class ParseNodeValueAdapter { 25 public: 26 ParseNodeValueAdapter(); 27 ~ParseNodeValueAdapter(); 28 get()29 const Value& get() { 30 if (ref_) 31 return *ref_; 32 return temporary_; 33 } 34 35 // Initializes the adapter for the result of the given expression. Returns 36 // truen on success. 37 bool Init(Scope* scope, const ParseNode* node, Err* err); 38 39 // Like Init() but additionally verifies that the type of the result matches. 40 bool InitForType(Scope* scope, 41 const ParseNode* node, 42 Value::Type type, 43 Err* err); 44 45 private: 46 // Holds either a reference to an existing item, or a temporary as a copy. 47 // If ref is non-null, it's valid, otherwise the temporary is used. 48 const Value* ref_; 49 Value temporary_; 50 51 ParseNodeValueAdapter(const ParseNodeValueAdapter&) = delete; 52 ParseNodeValueAdapter& operator=(const ParseNodeValueAdapter&) = delete; 53 }; 54 55 #endif // TOOLS_GN_PARSE_NODE_VALUE_ADAPTER_H_ 56