Question

我想初始化一个没有默认构造函数的模板大小的对象数组，如下面的代码所示：

#include <array>

template<std::size_t N>
class Foo
{
    public:
        class Bar
        {
                Foo<N> & _super;

            public:
                Bar(Foo<N> *super) :
                    _super(*super)
                {
                }
        };

        std::array<Bar, N>  _array;

        Foo(void) :
            _array{{}} // We need {this, ...} N times
        {
        }
};


int main(void)
{
    Foo<3>  foo;
    (void)foo;
    return 0;
}

这是一种说法：＆＃34;我想要一个N个对象的数组，所有这些对象都用相同的参数初始化＆＃34; ？我认为模板元编程有一种方法，但我无法想象如何去做。

Answer 1

在make_index_sequence：

的帮助下，一切皆有可能

   Foo() : Foo(std::make_index_sequence<N>()) {} 
   template <size_t... I> Foo(std::index_sequence<I...> ) : _array{((void)I, this)...} {}

注意_array构造函数中的逗号运算符（，） - 由@Quentin提供（与函数调用相反）。

Answer 2

您可以一次添加一个this，直到有N个_array为止，此时您只需初始化Foo() : Foo(this) { } private: template <class... T, std::enable_if_t<(sizeof...(T) < N), void*> = nullptr> Foo(T... args) : Foo(args..., this) { } template <class... T, std::enable_if_t<(sizeof...(T) == N), void*> = nullptr> Foo(T... args) : _array{{args...}} { }：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory2' defined in file [/home/pallav/91mobiles/community-api/dist/WEB-INF/classes/spring-config/spring-context.xml]: Invocation of init method failed; nested exception is org.hibernate.HibernateException: Missing table: Answer
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1568) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:540) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:956) ~[spring-context-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:747) ~[spring-context-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480) ~[spring-context-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:403) [spring-web-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:306) [spring-web-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106) [spring-web-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4811) [catalina.jar:8.0.33]
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5251) [catalina.jar:8.0.33]
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147) [catalina.jar:8.0.33]
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725) [catalina.jar:8.0.33]
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701) [catalina.jar:8.0.33]
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717) [catalina.jar:8.0.33]
    at org.apache.catalina.startup.HostConfig.deployDescriptor(HostConfig.java:587) [catalina.jar:8.0.33]
    at org.apache.catalina.startup.HostConfig$DeployDescriptor.run(HostConfig.java:1798) [catalina.jar:8.0.33]
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_77]
    at java.util.concurrent.FutureTask.run(FutureTask.java:266) [na:1.8.0_77]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_77]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_77]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_77]
Caused by: org.hibernate.HibernateException: Missing table: Answer
    at org.hibernate.cfg.Configuration.validateSchema(Configuration.java:1335) ~[hibernate-core-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.tool.hbm2ddl.SchemaValidator.validate(SchemaValidator.java:175) ~[hibernate-core-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:525) ~[hibernate-core-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1859) ~[hibernate-core-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl$4.perform(EntityManagerFactoryBuilderImpl.java:850) ~[hibernate-entitymanager-4.3.3.Final.jar:4.3.3.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl$4.perform(EntityManagerFactoryBuilderImpl.java:843) ~[hibernate-entitymanager-4.3.3.Final.jar:4.3.3.Final]
    at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.withTccl(ClassLoaderServiceImpl.java:398) ~[hibernate-core-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:842) ~[hibernate-entitymanager-4.3.3.Final.jar:4.3.3.Final]
    at org.hibernate.jpa.HibernatePersistenceProvider.createContainerEntityManagerFactory(HibernatePersistenceProvider.java:150) ~[hibernate-entitymanager-4.3.3.Final.jar:4.3.3.Final]
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:341) ~[spring-orm-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:318) ~[spring-orm-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1627) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1564) ~[spring-beans-4.1.1.RELEASE.jar:4.1.1.RELEASE]
    ... 25 common frames omitted

如何初始化模板大小的数组？

2 个答案: