我注意到函数模板中静态变量初始化的一个奇怪的行为.请考虑以下示例：
MyFile * createFile()
{
std::cout << "createFile" << std::endl;
return nullptr;
}
template <typename T>
void test(const T& t)
//void test(T t)
{
static MyFile *f = createFile();
}
void main()
{
test("one");
//test("two");
test("three");
}
只要测试中的f是静态的,我希望createFile只被调用一次.但是,它被调用两次.
花了一些时间来解决问题,我注意到从测试中的参数中删除const引用修复了它.另一个有趣的事情是传递给函数的字符串的长度也会影响初始化：当参数的长度相等时,静态变量只初始化一次,否则会发生新的初始化.
解决方法
文字“一”是一个const char [4].
这段代码：
test("one")
理想情况下会调用test(const char(&)[4])
这适用于测试(const T&)(因为const char(&)[4]可以绑定到const char(const&)[4]).
但它无法用于测试(T t),因为您无法按值传递字符串文字.它们通过引用传递.
但是,const char [4]可以衰减为const char *,它可以匹配模板< class T> void func(T t).
证据在于布丁：
#include <cstdint>
#include <iostream>
#include <typeinfo>
template <typename T,std::size_t N>
void test_const(const T(&t)[N])
{
std::cout << __func__ << " for literal " << t << " T is a " << typeid(T).name() << " and N is " << N << std::endl;
}
template <typename T>
void test_mutable(T &t)
{
std::cout << __func__ << " for literal " << t << " T is a " << typeid(T).name() << std::endl;
}
template <typename T>
void test_const_ref(const T &t)
{
std::cout << __func__ << " for literal " << t << " T is a " << typeid(T).name() << std::endl;
}
template <typename T>
void test_copy(T t)
{
std::cout << __func__ << " for literal " << t << " T is a " << typeid(T).name() << std::endl;
}
int main()
{
test_const("one");
test_const("three");
test_mutable("one");
test_mutable("three");
test_const_ref("one");
test_const_ref("three");
test_copy("one");
test_copy("three");
}
示例结果(clang)：
test_const for literal one T is a c and N is 4 test_const for literal three T is a c and N is 6 test_mutable for literal one T is a A4_c test_mutable for literal three T is a A6_c test_const_ref for literal one T is a A4_c test_const_ref for literal three T is a A6_c test_copy for literal one T is a PKc test_copy for literal three T is a PKc
这是一个带有demangled名称的版本(将在clang和gcc上编译)：
#include <cstdint>
#include <iostream>
#include <typeinfo>
#include <cstdlib>
#include <cxxabi.h>
std::string demangle(const char* name)
{
int status = -1;
// enable c++11 by passing the flag -std=c++11 to g++
std::unique_ptr<char,void(*)(void*)> res {
abi::__cxa_demangle(name,NULL,&status),std::free
};
return (status==0) ? res.get() : name ;
}
template <typename T,std::size_t N>
void test_const(const T(&t)[N])
{
std::cout << __func__ << " for literal " << t << " T is a " << demangle(typeid(T).name()) << " and N is " << N << std::endl;
}
template <typename T>
void test_mutable(T &t)
{
std::cout << __func__ << " for literal " << t << " T is a " << demangle(typeid(T).name()) << std::endl;
}
template <typename T>
void test_const_ref(const T &t)
{
std::cout << __func__ << " for literal " << t << " T is a " << demangle(typeid(T).name()) << std::endl;
}
template <typename T>
void test_copy(T t)
{
std::cout << __func__ << " for literal " << t << " T is a " << demangle(typeid(T).name()) << std::endl;
}
int main()
{
test_const("one");
test_const("three");
test_mutable("one");
test_mutable("three");
test_const_ref("one");
test_const_ref("three");
test_copy("one");
test_copy("three");
}
预期产量：
test_const for literal one T is a char and N is 4 test_const for literal three T is a char and N is 6 test_mutable for literal one T is a char [4] test_mutable for literal three T is a char [6] test_const_ref for literal one T is a char [4] test_const_ref for literal three T is a char [6] test_copy for literal one T is a char const* test_copy for literal three T is a char const*
