67 lines
3.2 KiB
Markdown
67 lines
3.2 KiB
Markdown
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# Const
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- **功能描述：** 指定该函数参数不可更改
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- **元数据类型：** bool
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- **引擎模块：** Blueprint, Parameter
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- **作用机制：** 在PropertyFlags中加入[CPF_ConstParm](../../../../Flags/EPropertyFlags/CPF_ConstParm.md)，在Meta中加入[NativeConst](../../../../Meta/Blueprint/NativeConst.md)
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- **常用程度：** ★
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指定该函数参数不可更改。
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如果在C++代码的参数上直接加const，则会自动的被UHT识别并添加CPF_ConstParm 标志，以及NativeConst元数据。但也可以手动加上UPARAM(const)来强制UHT添加CPF_ConstParm，效果见下面蓝图中的Out节点，把输出参数变成了输入参数。
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虽然不知道什么情况下需要手动添加，因此在源码中没有找到实际的用例。能想到的用处是在蓝图层面使它变成const输入参数，但是在C++层面依然是可变的引用参数，方便在C++里调用一些非const的方法。
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## 测试代码：
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```cpp
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//PropertyFlags: CPF_ConstParm | CPF_Parm | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_ConstInt(UPARAM(const) int value);
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//PropertyFlags: CPF_ConstParm | CPF_Parm | CPF_OutParm | CPF_ZeroConstructor | CPF_ReferenceParm | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_ConstIntOut(UPARAM(const) int& value);
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//(NativeConst = )
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//PropertyFlags: CPF_ConstParm | CPF_Parm | CPF_OutParm | CPF_ZeroConstructor | CPF_ReferenceParm | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_ConstIntRef(UPARAM(const) const int& value);
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//PropertyFlags: CPF_Parm | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_NoConstInt(int value);
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//PropertyFlags: CPF_Parm | CPF_OutParm | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_NoConstIntOut(int& value);
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//(NativeConst = )
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//PropertyFlags: CPF_ConstParm | CPF_Parm | CPF_OutParm | CPF_ZeroConstructor | CPF_ReferenceParm | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic
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UFUNCTION(BlueprintCallable)
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FString MyFuncTestParam_NoConstIntRef(const int& value);
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```
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## 蓝图节点：
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MyFuncTestParam_ConstIntOut的输出Value变成了输入的Value，因为不能改变。
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## 原理代码：
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在代码中实际出现const，就会增加CPF_ConstParam Flag。
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```cpp
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\Engine\Source\Programs\Shared\EpicGames.UHT\Parsers\UhtPropertyParser.cs 1030
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if (propertySettings.PropertyCategory != UhtPropertyCategory.Member && !isTemplateArgument)
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{
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// const before the variable type support (only for params)
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if (tokenReader.TryOptional("const"))
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{
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propertySettings.PropertyFlags |= EPropertyFlags.ConstParm;
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propertySettings.MetaData.Add(UhtNames.NativeConst, "");
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}
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}
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```
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