我一直在互联网上到处寻找，并没有找到任何体面的答案来解决问题。 代码：
<?php
if(ISSET($_POST['submit']){
$username =$_POST['uname'];
$password =$_POST['lpass'];
$mdpass = md5($password);
$result = mysql_query("select * from member where RegNo ='$username' and Password ='$mdpass'")or die(mysql_error());
$count =mysql_num_rows($result);
$row = mysql_fetch_array($result)
if($count>0){
session_start(); //start a session
$_session['id']=$row['id']; //create a session variable
header('location:home.php'); //redirect to home.php
}
else{
header('location:index.php'); //redirect to index.php
}
}
?>
最后一行给出以下错误：无法分配抽象类型'Song'的对象
虽然，A级覆盖了play（） - 方法，而且我正在分配A类的对象而不是P，我在这里做错了什么？
答案 0 :(得分：2)
问题在于您试图通过将对象复制到P myVar来A P P *myPtrVar = new A();
... // Use myPtrVar here
delete myPtrVar;
std::unique_ptr<P> mySmartPtrVar(new A());
中的其他实现。{/ p>
你不能用“价值”对象做到这一点;你需要一个指针或参考。基于内置C ++指针的解决方案如下所示：
delete基于智能指针的解决方案如下所示：
mySmartPtrVar使用slices off的好处是您无需调用using namespace std;
template <typename T, int N>
class BoundedBuffer {
private:
std::array<T, N> buffer;
int read_pos;
int write_pos;
std::mutex reader_mutex; //mutex for between readers
std::mutex writer_mutex; //mutex for between writers
std::mutex shared_mutex;
std::condition_variable reader_queue;
std::condition_variable writer_queue;
int timeout; //timeout in millisecond
public:
BoundedBuffer(const BoundedBuffer&) = delete;
BoundedBuffer& operator=(const BoundedBuffer&) = delete;
BoundedBuffer(int t) :
read_pos(0),
write_pos(0),
timeout(t) {
}
inline bool empty() {
return read_pos == write_pos;
}
inline bool full() {
return write_pos >= read_pos + N;
}
bool put(const T& data) {
unique_lock<mutex> writer_lock(writer_mutex);
{
unique_lock<mutex> shared_lock(shared_mutex);
if (full()) { //buffer full
if (writer_queue.wait_for(shared_lock, std::chrono::milliseconds(timeout)) ==
std::cv_status::timeout)
return false;
}
}
buffer[write_pos%N] = data;
write_pos++;
reader_queue.notify_one();
return true;
}
pair<T, bool> get() {
unique_lock<mutex> reader_lock(reader_mutex);
{
unique_lock<mutex> shared_lock(shared_mutex);
if (empty()) { //buffer empty
if (reader_queue.wait_for(shared_lock, std::chrono::milliseconds(timeout)) ==
std::cv_status::timeout) {
T t;
return make_pair(t, false);
}
}
}
pair<T, bool> result = make_pair(buffer[read_pos%N], true);
read_pos++;
writer_queue.notify_one();
return result;
}
};
：只要TextViews超出范围，对象就会被释放。