C++如何修复多声明编译器错误

Question

我有 2 个标题和 1 个 cpp 文件。 block .h:

    #ifndef BLOCK_H
    #define BLOCK_H

    namespace storage {

    class Block {
    };

    } // namespace storage

    #endif // BLOCK_H

PerformanceWriteTest.h

    #ifndef _PERFORMANCE_WRITE_TEST_
    #define _PERFORMANCE_WRITE_TEST_

    #include <string>
    #include <vector>

    using std::vector;

    class Block;  // <<< Forward declaration of Block

    class PerformanceWriteTest {
      vector<Block*> blocks_;
    public:
      virtual ~PerformanceWriteTest();
    };

    #endif

PerformanceWriteTest.cpp

    #include "Block.h"
    #include "PerformanceWriteTest.h"

    using storage::Block; // <<< Use the scope storage::Block. Error!

    PerformanceWriteTest::~PerformanceWriteTest() {
      for (Block* block : blocks_) {
        delete block;
      }
    }

Visual Studio 2012 给我错误:错误 C2874:使用声明导致“storage::Block”的多重声明

是否可以在不移动 using 指令并将“Block.h”包含到 header 中的情况下避免此错误？

Answer 1

问题是您声明了两个“ block ”。一个在命名空间“存储”内，另一个在全局命名空间中。试试这个:

namespace storage {
    class Block;  // <<< Forward declaration of Block
}
class PerformanceWriteTest {
   vector<storage::Block*> blocks_;
 public: