类x不存在默认构造函数，运算符新继承。

Question

我试图实现CUDA博客上给出的统一虚拟内存示例：http://devblogs.nvidia.com/parallelforall/unified-memory-in-cuda-6/

Visual 2013给出了一个错误:类"Term“不存在默认构造函数

#include "cuda_runtime.h"
#include "device_launch_parameters.h"

#include <stdio.h>
#include <iostream>

#define NUM_ATTRIBUTES 10

class Managed {
public:
    void *operator new(size_t len) {
      void *ptr;
      cudaMallocManaged(&ptr, len);
      std::cout << "custom new for size " << len << '\n';
      return ptr;
    }
    void *operator new[] (size_t len) {
      void *ptr;
      cudaMallocManaged(&ptr, len);
      std::cout << "custom new for size " << len << '\n';
      return ptr;
    }
    void operator delete(void *ptr) {
        cudaFree(ptr);
    }
};

class Attribute : public Managed {
public:
    int num_levels;
    int num_active_levels;
    int active_levels;
};

class Term : public Managed {
public:
    int num_active_attributes;
    int active_attributes;
    Attribute attribute[NUM_ATTRIBUTES];

    // Unified memory copy constructor allows pass-by-value
    Term (const Term &orig) {
        num_active_attributes = orig.num_active_attributes;
        active_attributes = orig.active_attributes;
        cudaMallocManaged((void **)&attribute,NUM_ATTRIBUTES * sizeof(Attribute));
        memcpy(attribute, orig.attribute, NUM_ATTRIBUTES * sizeof(Attribute));
    }
};

int main()
{
    Term * A = new Term[10];
    getchar();
    return 0;
}

类术语不是继承自在管理的父类中定义的新运算符吗？

Answer 1

这与数据自动化系统无关。编译器没有生成默认构造函数，因为您已经提供了另一个构造函数(复制构造函数)。

只需显式请求默认构造函数：

class Term : public Managed {
public:
    ...

    // C++11
    Term() = default;

    // Pre C++11 alternative (although not strictly equivalent)
    Term() {}
    ...
};

还请注意，operator new和构造函数是两件不同的事情：new用于分配存储，构造函数用于将该存储初始化为有意义的状态(大致如此)。

Answer 2

您需要定义数组元素所需的默认构造函数。

class Term : public Managed {
public:
    //...
    Term () {}  // default c-tor
    Term() = default;  // since C++11, note however that there are some 
                       // differences between Term() = default; and Term() {}
                       // they are not equal when value initialization 
                       // is considered
};

因为显式定义的复制-ctor Term( const Term&)禁止编译器生成默认构造函数，所以没有。关于=default和{}在默认构造函数方面的差异，您可以查看以下内容：

https://stackoverflow.com/a/23698999/1141471