类和main内部的std :: unique_ptr定义

Question

为什么std::unique_ptr<A> pA1(new A);中的main()定义可以，但是在B类定义中却产生错误？

#include <iostream>
#include <string>
#include <memory>

class A {
public:
    A() { std::cout << "A::A\n"; }
    ~A() { std::cout << "A::~A\n"; }
};

class B
{    
public:
    int i = 12;
    std::unique_ptr<A> pA(new A); // Fail
    //std::unique_ptr<A> pA = new A(); // Also fail

    virtual ~B() noexcept {
        std::cout << "B::~B\n";
    }

    void f1() {
        std::cout << i << std::endl;
    }
};

int main()
{
    std::unique_ptr<A> pA1(new A); // Ok
    B b = B();
    b.f1();
}

g ++编译器实际说的是：g++ -std=c++11 example_1.cpp

example_1.cpp:15:27: error: expected parameter declarator
    std::unique_ptr<A> pA(new A); // Fail
                          ^
example_1.cpp:15:27: error: expected ')'
example_1.cpp:15:26: note: to match this '('
    std::unique_ptr<A> pA(new A); // Fail
                         ^
2 errors generated.

好像第一个错误说它将pA解释为函数？