在MFC、C++及JNI中调用方法时::的含义与作用

Hey there! Let's break down what that double colon (::) is doing in your code, since it pops up in a few different contexts you mentioned. It's a core C++ operator, so its behavior stays consistent across MFC and JNI—though the use cases might look a bit different depending on the framework.

In Standard C++ and MFC #

First off, :: is called the scope resolution operator. Its job is to tell the compiler exactly where to look for a variable, function, class, or member—essentially cutting through ambiguity and specifying which "scope" the item lives in. Here are the most common uses you'll see:

• Accessing global scope items: If you have a local variable with the same name as a global one, :: lets you reference the global version. For example:

  int counter = 0; // Global variable
  void UpdateCounter() {
      int counter = 5; // Local variable
      ::counter += 1; // Modifies the GLOBAL counter, not the local one
  }
  

• Calling static class members: Static methods or variables belong to the class itself, not individual objects. You use :: to call them without an instance:

  // MFC example: Calling a static method of CString
  int strLength = CString::GetLength(_T("Hello MFC"));
  

• Referencing items in a namespace: Namespaces group related code to avoid naming clashes. :: lets you pull items out of a namespace:

  // Standard C++ example
  std::vector<int> myVector;
  // Or if you don't have a 'using namespace std;' line:
  ::std::vector<int> anotherVector;
  

In MFC specifically, you'll see :: all the time to reference framework classes (like CWnd::Create()) or global MFC functions (like ::AfxMessageBox())—it's just a way to be explicit about what you're calling, especially when working with large codebases where name collisions are possible.

In JNI Code #

When you're writing JNI (Java Native Interface) code in C++, :: still acts as the scope resolution operator—there's no special JNI-specific behavior here. The main places you'll encounter it are:

• Accessing JNI class members: The JNI header (jni.h) defines classes like JNIEnv and JavaVM. When working with the C++ version of JNI (not the C version), you might use :: to reference static methods or nested types from these classes:

  // Get the JNIEnv for the current thread
  JNIEnv* env;
  javaVM->GetEnv((void**)&env, JNI_VERSION_1_6);
  

• Referencing your own code's scopes: If you've organized your JNI helper functions into namespaces or classes, :: lets you call them clearly. For example:

  namespace jni_utils {
      jstring ConvertToJString(JNIEnv* env, const char* str) {
          return env->NewStringUTF(str);
      }
  }
  // Call the helper function from elsewhere in your JNI code
  jstring jStr = jni_utils::ConvertToJString(env, "Hello JNI");
  

It's worth noting that the C version of JNI uses function pointers instead of classes, so you won't see :: there—you'll use syntax like (*env)->FindClass(env, "...") instead. But in C++ JNI code, :: follows the same rules as standard C++.

Is :: Related to Multithreading? #

Short answer: No, not directly. The scope resolution operator has nothing to do with thread safety, thread creation, or any multithreading mechanics.

That said, you might see :: used alongside multithreading code—for example:

• In MFC, when starting a thread with AfxBeginThread, you might pass a global thread function using ::MyThreadFunc to make sure you're referencing the global function, not a local one.
• In JNI, when retrieving a JNIEnv for a new thread (since each thread has its own JNIEnv), you'll call JavaVM::GetEnv()—here, :: is just accessing the static method of the JavaVM class, not handling the thread logic itself.

The :: operator's only job is to resolve scope—any connection to multithreading is just a coincidence of how you're structuring your code in a multithreaded context.

内容的提问来源于stack exchange，提问作者Vivek S.