Item2 理解auto 的类型推导
auto 的类型推导与template 的类型推导类似，主要的区别在于对于 std::initialize_list 的区别。
在这个子项中的类型推导我们用到了boost 的1.67的版本。
具体的代码如下:
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#include <iostream>
#include <typeinfo>
#include <boost/type_index.hpp>
void func(int,double)
{
}
int main(int argc,char * argv[])
{
using boost::typeindex::type_id_with_cvr;
//Case1:type specifier is a pointer or a reference but not a universal reference.
auto x = 27;
const auto cx = x;
const auto& crx = x;
auto& rx = x;
std::cout << "x Type is " << type_id_with_cvr<decltype(x)>().pretty_name() << std::endl;
std::cout << "cx Type is " << type_id_with_cvr<decltype(cx)>().pretty_name() << std::endl;
std::cout << "crx Type is " << type_id_with_cvr<decltype(crx)>().pretty_name() << std::endl;
std::cout << "rx Type is " << type_id_with_cvr<decltype(rx)>().pretty_name() <<std::endl;
auto&& uref1 = x;
auto&& uref2 = cx;
auto&& uref3 = rx;
std::cout << "uref1 Type is " << type_id_with_cvr<decltype(uref1)>().pretty_name() << std::endl;
std::cout << "uref2 Type is " << type_id_with_cvr<decltype(uref2)>().pretty_name() << std::endl;
std::cout << "uref3 Type is " << type_id_with_cvr<decltype(uref3)>().pretty_name() << std::endl;
const char name[]="Hello Dennis";
auto arr1 = name;
auto& arr2 = name;
std::cout << "name Type is " << type_id_with_cvr<decltype(name)>().pretty_name() << std::endl;
std::cout << "arr1 Type is " << type_id_with_cvr <decltype(arr1)>().pretty_name() << std::endl;
std::cout << "arr2 Type is " << type_id_with_cvr<decltype(arr2)>().pretty_name() << std::endl;
auto func1 = func;
auto& func2 = func;
auto lamdaFunc = []()->void {return;};
std::cout << "func1 Type is " <<type_id_with_cvr<decltype(func)>().pretty_name() << std::endl;
std::cout << "func1 Type is " << type_id_with_cvr<decltype(func1)>().pretty_name() << std::endl;
std::cout << "func2 Type is " <<type_id_with_cvr<decltype(func2)>().pretty_name() << std::endl;
std::cout << "lamdaFunc Type is "<< type_id_with_cvr<decltype(lamdaFunc)>().pretty_name() << std::endl;
int x1 = 27;
int x2(27);
int x3={27};
int x4{27};
std::cout << "x1 Type is " << type_id_with_cvr<decltype(x1)>().pretty_name() <<std::endl;
std::cout << "x2 Type is " << type_id_with_cvr<decltype(x2)>().pretty_name() <<std::endl;
std::cout << "x3 Type is " << type_id_with_cvr<decltype(x3)>().pretty_name() << std::endl;
std::cout <<"x4 Type is "<< type_id_with_cvr<decltype(x4)>().pretty_name() << std::endl;
auto ax1 = 27;
auto ax2(27);
auto ax3={27};
auto ax4{27};
std::cout << "ax1 Type is " << type_id_with_cvr<decltype(ax1)>().pretty_name() << std::endl;
std::cout << "ax2 Type is " << type_id_with_cvr<decltype(ax2)>().pretty_name() << std::endl;
std::cout << "ax3 Type is " << type_id_with_cvr<decltype(ax3)>().pretty_name() << std::endl;
std::cout <<"ax4 Type is " <<type_id_with_cvr<decltype(ax4)>().pretty_name() << std::endl;
return 0;
}
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程序的输出如下:
使用 clang6.0 编译。
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x Type is int
cx Type is int const
crx Type is int const&
rx Type is int&
uref1 Type is int&
uref2 Type is int const&
uref3 Type is int&
name Type is char const [13]
arr1 Type is char const*
arr2 Type is char const (&) [13]
func1 Type is void (int, double)
func1 Type is void (*)(int, double)
func2 Type is void (&)(int, double)
lamdaFunc Type is main::$_0
x1 Type is int
x2 Type is int
x3 Type is int
x4 Type is int
ax1 Type is int
ax2 Type is int
ax3 Type is std::initializer_list int>
ax4 Type is int
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两者的区别主要在于 std::initializer_list
例如:
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//代码1
template<typename T>
void func(T param)
{
}
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对于形如代码1的代码，是不可以这样调用的func({1,2,3});
如果要实现上面的调用方式，需要进行如下的实现
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template <typename T>
void func(std::initailizer_list T> param);
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总结:
auto 的类型推导和template 类似，区别在于auto 默认由花括号{} 的初始化代表一个 std::initializer_list ，而模板的类型推导不是这样的。- 用在函数返回值的
auto 实现的是模板类型推导，而不是auto类型推导。