C++ :: Set value 函数在函数内部设置，但后面跟着 Get 函数，设置值消失了

Question

希望我遗漏了一些非常明显的东西：

尝试使用一些向量作为容器，使用资产属性修饰符 (class) 设置资产 (class) 的值。这似乎在函数调用期间有效，但是当在 set 调用之后进行 get 调用以确认时，就好像 set 调用没有发生一样。请参阅下面的代码和输出：

asset.h 文件：

namespace assets {
class Asset 
{
  friend class AssetPropertyModifier;
  std::vector<properties::AssetPropertyDouble> asset_property_double_;
  enum property_id_ { ZERO, USHORT, FLOAT, INT, UINT, DOUBLE, VEC3, STRING, 
       BOOL};
public:
  void AddProperty(std::string property_name, double property_value);
};//class Asset
}//namespace assets

AddProperty 工作正常，所以我忽略了它和 .cpp 不相关。只需将 push_back 添加到向量中以添加值，以及一些索引跟踪。

在资产中属性header:

namepsace properties {
struct AssetPropertyDouble
{
private:
  friend class AssetPropertyModifier;
  double real_number_value_;
public:
  AssetPropertyDouble(double d) : real_number_value_(d) {}
};//struct AssetPropertyDouble
}//namespace proeprties

这是 APM header:

namespace assets {
class Asset;
class AssetPropertyModifier
{
  int LookUpVectorIndex(Asset a, int type , int index);
  int LookUpNameIndex(Asset a, std::string s);
public:
  double GetPropertyValue(Asset a, std::string property_name, 
         double not_used_just_an_overloader);
  void SetPropertyValue(Asset a, std::string property_name, 
       double new_double_value);
};//class AssetPropertyModifier
}//namespace assets

在 APM .cpp 文件中：

namespace assets {

//LookUpNameIndex definition
//LookUpVectorIndex defintion

double AssetPropertyModifier::GetPropertyValue(Asset a, 
                              std::string property_name, 
                              double not_used_just_an_overloader)
{
  //Using Verbose output to debug:
  std::cout << "------------GET-------------------" << std::endl;
  int name_index = LookUpNameIndex(a, property_name);
  std::cout << "names vector index is " << name_index << std::endl;
  int double_index = LookUpVectorIndex(a, a.DOUBLE, name_index);
  std::cout << "double property vector index is " << double_index << 
       std::endl;
  double property_value = 
       a.asset_property_double_.at(double_index).real_number_value_;
  std::cout << property_name << " are equal to " << property_value << 
       std::endl;
  return property_value;
}

void AssetPropertyModifier::SetPropertyValue(Asset a, 
                            std::string property_name, 
                            double new_double_value)
{
   //Using Verbose output to debug:
  std::cout << "------------SET-------------------" << std::endl;
  int name_index = LookUpNameIndex(a, property_name);
  std::cout << "names vector index is " << name_index << std::endl;
  int double_index = LookUpVectorIndex(a, a.DOUBLE, name_index);
  std::cout << "double vector index is " << double_index << std::endl;
  a.asset_property_double_.at(double_index).real_number_value_ = 
       new_double_value;
  std::cout << property_name << " was changed to " <<
        a.asset_property_double_.at(double_index).real_number_value_
        << std::endl;
}//SetPropertyValue

}//namespace assets

然后当我使用这个原型时，会发生以下情况：

来自 main.cpp 示例：

std::string some_name = "Property Name";
double some_initial_value = 5.5;
double some_new_value = 4.4;
assets::AssetPropertyModifier apm;
assets::Asset my_asset;

main {
  my_asset.AddProperty(some_name, some_initial_value);
  std::cout << "Current value = " << apm.GetPropertyValue(my_asset, 
       some_name , _DOUBLE_)
       << std::endl;

  apm.SetPropertyValue(my_asset, some_name , some_new_value );

  std::cout << "Current value = " << apm.GetPropertyValue(my_asset, 
       some_name , _DOUBLE_)
       << std::endl;
}

这是结果输出：

------------GET-------------------
names vector index is 0
double property vector index is 0
Property Name are equal to 5.5
Current value = 5.5
------------SET-------------------
names vector index is 0
double vector index is 0
Property Name was changed to 4.4
------------GET-------------------
names vector index is 0
double property vector index is 0
Property Name are equal to 5.5
Current value = 5.5

如您所见，GET 返回 5.5，SET 声明它更新为 4.4，但随后的 GET 仍显示 5.5。

我可以提供的另一件事来帮助解决这个问题，如果我将属性设置为 public 以便我可以直接从 main 访问资产属性以进行测试，它正确更新：

//hard SET with public access to the vector and the property struct value
my_asset.asset_property_double_.at(0).real_number_value_ = 3.3;
//hard GET with public access:
std::cout << my_asset.asset_property_double_.at(0).real_number_value_ << 
     std::endl;
//GET through APM but with public values still enabled:
std::cout << "Current value = " << apm.GetPropertyValue(my_asset, some_name, 
     _DOUBLE_)
    << std::endl;

那个输出是：

3.3
------------GET-------------------
names vector index is 0
double property vector index is 0
Property Name are equal to 3.3
Current value = 3.3

有谁知道为什么SET函数没有粘在上面？在此先感谢您的帮助！

Answer 1

您正在传递资产的副本而不是对资产的引用，因此您实际上只是在更新副本，而不是实际的。您将需要通过引用传递：

void AssetPropertyModifier::SetPropertyValue(Asset& a,        //<----added '&'
                            std::string property_name, 
                            double new_double_value)
{
   //Using Verbose output to debug:
  std::cout << "------------SET-------------------" << std::endl;
  int name_index = LookUpNameIndex(a, property_name);
  std::cout << "names vector index is " << name_index << std::endl;
  int double_index = LookUpVectorIndex(a, a.DOUBLE, name_index);
  std::cout << "double vector index is " << double_index << std::endl;
  a.asset_property_double_.at(double_index).real_number_value_ = 
       new_double_value;
  std::cout << property_name << " was changed to " <<
        a.asset_property_double_.at(double_index).real_number_value_
        << std::endl;
}//SetPropertyValue

C++ :: Set value 函数在函数内部设置，但后面跟着 Get 函数，设置值消失了

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