0 
#include <iostream>
#include <vector>

using namespace std;


class block{
public:
    long nx,ny;
    vector<long> s;
    block(long &x, long &y):nx(x),ny(y),vector<long> s((x+1)*(y+1),0) {}
};

int main() {
    block B(2,2);
    for(int i=1;i<=9;i++) {
        cout<<B.s(i);
    }

    cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!
    return 0;
}

编译后错误信息显示符号“vector”无法解析。错误是什么？我想定义一个包含要初始化的可变维度向量的类。

#include <iostream>
#include <vector>

using namespace std;


class block{
public:
    long nx,ny;
    vector<long> s;
    block(long &x, long &y):nx(x),ny(y),s((x+1)*(y+1),0) {}
};

int main() {
    block B(2,2);
    for(int i=0;i<=9;i++) {
        cout<<B.s[i];
    }

    cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!
    return 0;
}

在块 B(2,2) 处仍有问题；错误消息：构造函数“block::block”的实例与参数列表不匹配

为什么？谢谢！

4

2 回答 2

4

Firstly:

  vector<long> s((x+1)*(y+1),0)

should be:

  s((x+1)*(y+1),0)

should not repeat the type for s. Meanwhile the constructor should be:

  block(const long &x, const long &y):
    nx(x), ny(y), s((x + 1) * (y + 1), 0)
  {
  }

if you really need reference. Since otherwise, when you do 

 block B(2,2);

inside main, it will give you error because constructor takes long&, you are passing int constants. Deep reason about this is related to lvalue and rvalues: integer constants are rvalues, however, long& is reference to non-const long, which is lvalue reference. According to this blog: lvalues, rvalues and references

An lvalue reference (to non-const type) is a reference that can be initialized with an lvalue. Well, only with those lvalues that do not render const or volatile types. An rvalue reference (to non-const type) is a reference that can be initialized with an rvalue (again, only with those rvalues that do not designate const or volatile types). An lvalue reference to const type is a reference that can be initialized with rvalues and lvalues alike (rendering constant and non-constant types).

Further, according to C++11 standard: Section 4.1 standard conversions:

Standard conversions are implicit conversions defined for built-in types. A standard conversion sequence is a sequence of standard conversions in the following order:

— Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.

— Zero or one conversion from the following set: integral promotions, floating point promotion, integral conversions, floating point conversions, floating-integral conversions, pointer conversions, pointer to member conversions, and boolean conversions.

— Zero or one qualification conversion.

There is no rvalue-to-lvalue conversion. This is why you saw that compile error. Adding const before long&make it be able to be initialized with rvalues, that's why the error goes away after the change.

Secondly,

  cout<<B.s(i);

should be:

cout<<B.s[i];

You should use [] to access vector elements.

Thirdly, vector index starts from 0, so 

for(int i=1;i<=9;i++)

should be

for(int i=0;i<9;i++)

Otherwise, index out of bounds. See a working example here: http://ideone.com/YLT3mG

于 2013-05-10T02:39:32.857 回答
3 
block(long &x, long &y):nx(x),ny(y),vector<long> s((x+1)*(y+1),0) {}

Don't repeat vector<long> in the initialization list. There's also no reason to pass in references, so delete the ampersands.

block(long x, long y):
    nx(x), ny(y), s((x + 1) * (y + 1), 0)
{
}
于 2013-05-10T02:38:33.060 回答