在C ++中初始化简单布尔向量失败

Question

I am trying to initialize a simple vector with false values and then use it. I rellized the value is never 0 or 1, so I printed it. The result is that even in the beginning it has strange big values. I am compiling with g++ (GCC) 4.4.7. The question refers to printing only vector data of type bool.

What I did:

std::vector<bool> n;
int f = 0;
for(f = 0; f<10; f++)
    n.push_back(false);
for(f = 0; f<10; f++)
    printf("content of %d %d",f,n[f]);

What I got:

content of 0 30572784
content of 1 30572784
content of 2 30572784
content of 3 30572784
content of 4 30572784
content of 5 30572784
content of 6 30572784
content of 7 30572784
content of 8 30572784
content of 9 30572784

What am I doing wrong?

Answer 1

要初始化布尔向量，可以使用文档http://www.cplusplus.com/reference/vector/vector/vector/中给出的fill构造函数

std::vector<bool> n(10, false); (*)

Answer 2

The problem is %d is for int (32bits), but to make a vector of bool more space efficient, vector<bool> is a specialized class that stores a bool as a single bit by using a proxy class. And, operator [] actually returns a reference of that proxy class ( http://en.cppreference.com/w/cpp/container/vector_bool )

If you want to use printf , you'd need to cast it to a bool first

for(f = 0; f<10; f++)
    printf("content of %d %d",f,bool(n[f]));

Or, as others have mentioned in the Comments, use cout in C++.

cout << "content of " << f <<" " << n[f];

If you want to see true/false, you can also use std::boolalpha

cout << boolalpha << "content of " << f <<" " << n[f]; //will see true/false