为什么函数参数的行为类似于变量声明？

Question

我在使用 Unreal 和运行时出现了一些奇怪的错误。我最终发现只需重命名我的函数参数就可以解决问题。这是完整的 header 和 class。问题是 InitCard 函数：



#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "Components/StaticMeshComponent.h"
#include "Playing_Card.generated.h"

UCLASS()
class BLACKJACK_API APlaying_Card : public AActor
{
    GENERATED_BODY()

    
public: 
    enum Suit { HEARTS = 0, DIAMONDS = 1, CLUBS = 2, SPADES = 3, UNASSIGNED = 4 };
    Suit suit = Suit::UNASSIGNED;

    // Sets default values for this actor's properties
    APlaying_Card();

    int rank;
    int value;

    void InitCard(int rank, Suit suit);
    
    FString faceStr = "";

    UPROPERTY(VisibleAnywhere, BlueprintReadWrite)
    UStaticMeshComponent* mesh;


protected:
    // Called when the game starts or when spawned
    virtual void BeginPlay() override;

public: 
    // Called every frame
    virtual void Tick(float DeltaTime) override;

};

Playing_Card.cpp:




#include "Playing_Card.h"


int rank = 0;
int value = 0;

// Sets default values
APlaying_Card::APlaying_Card()
{
    // Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
    PrimaryActorTick.bCanEverTick = true;

    mesh = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("mesh"));

    RootComponent = mesh;

    mesh->SetScalarParameterValueOnMaterials(TEXT("card_index"), 4);    
}



void APlaying_Card::InitCard(int rank, Suit suit)
{
    this->rank = rank;
    this->suit = suit;

    if (rank == 1)
    {
        value = rank; //todo: find a way to make Ace 1 or 11.
        faceStr = "Ace";
    }
    else if (rank <= 10)
    {
        value = rank;
        faceStr = FString::FromInt(rank);
    }
    else if (rank == 11)
    {
        value = 10;
        faceStr = "Jack";
    }
    else if (rank == 12)
    {
        value = 10;
        faceStr = "Queen";
    }
    else if (rank == 13)
    {
        value = 10;
        faceStr = "King";
    }

}

// Called when the game starts or when spawned
void APlaying_Card::BeginPlay()
{
    Super::BeginPlay();
    
}

// Called every frame
void APlaying_Card::Tick(float DeltaTime)
{
    Super::Tick(DeltaTime);

}

这给出了错误： error C4458: declaration of 'rank' hides class member

但是，如果我更改函数参数名称（下面发布的函数），代码编译时不会出错。

    /// It works! Just by changing parameter names (note: I still use this->rank)
void APlaying_Card::InitCard(int cardRank, Suit cardSuit)
{
    this->rank = cardRank;
    this->suit = cardSuit;

    if (rank == 1)
    {
        value = rank; //todo: find a way to make Ace 1 or 11.
        faceStr = "Ace";
    }
    else if (rank <= 10)
    {
        value = rank;
        faceStr = FString::FromInt(rank);
    }
    else if (rank == 11)
    {
        value = 10;
        faceStr = "Jack";
    }
    else if (rank == 12)
    {
        value = 10;
        faceStr = "Queen";
    }
    else if (rank == 13)
    {
        value = 10;
        faceStr = "King";
    }

}

第一次尝试使用名称时出现错误的原因是什么？

Answer 1

原代码中，全局变量rank/suit、class成员rank/suit、参数rank/suit 都在 InitCard() 的范围内。那么您希望编译器在语句 this->rank = rank; this->suit = suit; 的右侧使用哪些？它不会是 class 成员，而是函数参数，因为它们在本地范围内。编译器会警告您这一点，因为它们与 class 成员同名。

删除全局变量并重命名参数，避免了任何歧义。因此，使用其 member initialization list 在其构造函数中初始化 class，而不是单独的 Init 方法：

UCLASS()
class BLACKJACK_API APlaying_Card : public AActor
{
...
public: 
    ...
    APlaying_Card(int rank, Suit suit);
    ...
};

APlaying_Card::APlaying_Card(int rank, Suit suit) :
    rank(rank), suit(suit) // <-- no ambiguity here! 
{
    ...

    if (rank == 1)
    {
        value = rank; //todo: find a way to make Ace 1 or 11.
        faceStr = "Ace";
    }
    else if (rank <= 10)
    {
        value = rank;
        faceStr = FString::FromInt(rank);
    }
    else if (rank == 11)
    {
        value = 10;
        faceStr = "Jack";
    }
    else if (rank == 12)
    {
        value = 10;
        faceStr = "Queen";
    }
    else if (rank == 13)
    {
        value = 10;
        faceStr = "King";
    }
}

Answer 2

不是因为参数的行为像一个变量声明（尽管它们只是初始化方式不同），而是因为只是名称重用（它可能不仅仅是一个已经存在的变量的名称).

在 C++ 语言方面，按照最初的方式命名参数是完全正确的，但在代码设计方面很容易出错，因为很容易误用名称，从而得到与预期不同的结果。

最初，C4458 是一个警告，编译器会通过该警告通知您您做了这种容易出错的事情，但不是错误 - 因此通常它不会阻止成功编译程序；但问题的症结在于您的编译器将警告视为错误，它必须根据相应的编译器设置发生；如果需要，您可以关闭它。

为什么函数参数的行为类似于变量声明？

Why is a function parameter behaving like a variable declaration?

c++

unreal-engine4