如何：在 C++/CLI 中使用跟踪引用

项目
04/03/2023

本文介绍如何使用 C++/CLI 中的跟踪引用 (%) 按引用传递公共语言运行时 (CLR) 类型。

按引用传递 CLR 类型

下面的示例演示如何使用跟踪引用按引用传递 CLR 类型。

// tracking_reference_handles.cpp
// compile with: /clr
using namespace System;

ref struct City {
private:
   Int16 zip_;

public:
   City (int zip) : zip_(zip) {};
   property Int16 zip {
      Int16 get(void) {
         return zip_;
      }   // get
   }   // property
};

void passByRef (City ^% myCity) {
   // cast required so this pointer in City struct is "const City"
   if (myCity->zip == 20100)
      Console::WriteLine("zip == 20100");
   else
      Console::WriteLine("zip != 20100");
}

ref class G {
public:
   int i;
};

void Test(int % i) {
   i++;
}

int main() {
   G ^ g1 = gcnew G;
   G ^% g2 = g1;
   g1 -> i = 12;

   Test(g2->i);   // g2->i will be changed in Test2()

   City ^ Milano = gcnew City(20100);
   passByRef(Milano);
}

zip == 20100

下一个示例演示如何获取跟踪引用的地址返回 interior_ptr (C++/CLI) ，并演示如何通过跟踪引用修改和访问数据。

// tracking_reference_data.cpp
// compile with: /clr
using namespace System;

public ref class R {
public:
   R(int i) : m_i(i) {
      Console::WriteLine("ctor: R(int)");
   }

   int m_i;
};

class N {
public:
   N(int i) : m_i (i) {
      Console::WriteLine("ctor: N(int i)");
   }

   int m_i;
};

int main() {
   R ^hr = gcnew R('r');
   R ^%thr = hr;
   N n('n');
   N %tn = n;

   // Declare interior pointers
   interior_ptr<R^> iphr = &thr;
   interior_ptr<N> ipn = &tn;

   // Modify data through interior pointer
   (*iphr)->m_i = 1;   // (*iphr)->m_i == thr->m_i
   ipn->m_i = 4;   // ipn->m_i == tn.m_i

   ++thr-> m_i;   // hr->m_i == thr->m_i
   ++tn. m_i;   // n.m_i == tn.m_i

   ++hr-> m_i;   // (*iphr)->m_i == hr->m_i
   ++n. m_i;   // ipn->m_i == n.m_i
}

ctor: R(int)
ctor: N(int i)

跟踪引用和内部指针

下面的代码示例演示如何在跟踪引用和内部指针之间进行转换。

// tracking_reference_interior_ptr.cpp
// compile with: /clr
using namespace System;

public ref class R {
public:
   R(int i) : m_i(i) {
      Console::WriteLine("ctor: R(int)");
   }

   int m_i;
};

class N {
public:
   N(int i) : m_i(i) {
      Console::WriteLine("ctor: N(int i)");
   }

   int m_i;
};

int main() {
   R ^hr = gcnew R('r');
   N n('n');

   R ^%thr = hr;
   N %tn = n;

   // Declare interior pointers
   interior_ptr<R^> iphr = &hr;
   interior_ptr<N> ipn = &n;

   // Modify data through interior pointer
   (*iphr)->m_i = 1;   // (*iphr)-> m_i == thr->m_i
   ipn->m_i = 4;   // ipn->m_i == tn.m_i

   ++thr->m_i;   // hr->m_i == thr->m_i
   ++tn.m_i;   // n.m_i == tn.m_i

   ++hr->m_i;   // (*iphr)->m_i == hr->m_i
   ++n.m_i;   // ipn->m_i == n.m_i
}

ctor: R(int)
ctor: N(int i)

使用跟踪引用和值类型

此示例通过对值类型的跟踪引用显示简单的装箱：

// tracking_reference_valuetypes_1.cpp
// compile with: /clr

using namespace System;

int main() {
   int i = 10;
   int % j = i;
   Object ^ o = j;   // j is implicitly boxed and assigned to o
}

下一个示例显示演示如何同时跟踪对值类型的引用和本机引用。

// tracking_reference_valuetypes_2.cpp
// compile with: /clr
using namespace System;
int main() {
   int i = 10;
   int & j = i;
   int % k = j;
   i++;   // 11
   j++;   // 12
   k++;   // 13
   Console::WriteLine(i);
   Console::WriteLine(j);
   Console::WriteLine(k);
}

13
13
13

下面的示例演示如何将跟踪引用与值类型和本机类型结合使用。

// tracking_reference_valuetypes_3.cpp
// compile with: /clr
value struct G {
   int i;
};

struct H {
   int i;
};

int main() {
   G g;
   G % v = g;
   v.i = 4;
   System::Console::WriteLine(v.i);
   System::Console::WriteLine(g.i);

   H h;
   H % w = h;
   w.i = 5;
   System::Console::WriteLine(w.i);
   System::Console::WriteLine(h.i);
}

此示例演示如何将跟踪引用绑定到垃圾回收堆上的值类型：

// tracking_reference_valuetypes_4.cpp
// compile with: /clr
using namespace System;
value struct V {
   int i;
};

void Test(V^ hV) {   // hv boxes another copy of original V on GC heap
   Console::WriteLine("Boxed new copy V: {0}", hV->i);
}

int main() {
   V v;   // V on the stack
   v.i = 1;
   V ^hV1 = v;   // v is boxed and assigned to hV1
   v.i = 2;
   V % trV = *hV1;   // trV is bound to boxed v, the v on the gc heap.
   Console::WriteLine("Original V: {0}, Tracking reference to boxed V: {1}", v.i, trV.i);
   V ^hV2 = trV;   // hv2 boxes another copy of boxed v on the GC heap
   hV2->i = 3;
   Console::WriteLine("Tracking reference to boxed V: {0}", hV2->i);
   Test(trV);
   v.i = 4;
   V ^% trhV = hV1;  // creates tracking reference to boxed type handle
   Console::WriteLine("Original V: {0}, Reference to handle of originally boxed V: {1}", v.i, trhV->i);
}

Original V: 2, Tracking reference to boxed V: 1
Tracking reference to boxed V: 3
Boxed new copy V: 1
Original V: 4, Reference to handle of originally boxed V: 1

采用本机、值或引用参数的函数模板

通过在函数模板的签名中使用跟踪引用，可确保该函数可由类型为本机、CLR 值或 CLR 引用的参数调用。

// tracking_reference_template.cpp
// compile with: /clr
using namespace System;

class Temp {
public:
   // function templates
   template<typename T>
   static int f1(T% tt) {   // works for object in any location
      Console::WriteLine("T %");
      return 0;
   }

   template<typename T>
   static int f2(T& rt) {   // won't work for object on the gc heap
      Console::WriteLine("T &");
      return 1;
   }
};

// Class Definitions
ref struct R {
   int i;
};

int main() {
   R ^hr = gcnew R;
   int i = 1;

   Temp::f1(i); // ok
   Temp::f1(hr->i); // ok
   Temp::f2(i); // ok

   // error can't track object on gc heap with a native reference
   // Temp::f2(hr->i);
}

T %
T %
T &

另请参阅

跟踪引用运算符

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