Array in template

本文讨论在array在template中的一些列问题。

Parameter passing

Function template的template parameter的template argument是一个array，在这种情况下就涉及到array的parameter passing问题，在前面的章节已知，array的parameter passing的方式有:

1) by value: array to pointer decay

2) by reference

本文将结合具体的例子来对这两种方式进行分析。

stackoverflow why does the array decay to a pointer in a template function

I don't understand why the array decays to a pointer in a template function.

NOTE: 这篇文章的原问题所讨论的是"array decay to a pointer"，在 stackoverflow How do I use arrays in C++? 中已经对此进行了探讨；我所关注的将array用于function template时，可以使用的两种 parameter passing的写法，以及两种写法的差异；

我是在阅读 stackoverflow What is std::decay and when it should be used? 的回答的**评论**:

It is array decay. For example: template<typename T> void f(T&); f("abc"); T is char(&)[4], but template<typename T> void f(T); f("abc"); T is char*; You can also find an explanation here: stackoverflow.com/questions/7797839/…

发现的这篇文章。

显然，当pass-by-reference的时候，是不会发生 array to pointer decay 的。上面的评论已经描述了这种情况了。

#include <iostream>

template<class T>
void f(T buff)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    std::cout << "f:buff size:" << sizeof(buff) << std::endl;       //prints 4
}

template<class T>
void f1(T &buff)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    std::cout << "f:buff size:" << sizeof(buff) << std::endl;       //prints 3
}

int main(int argc, char *argv[])
{
    const char buff[3] = { 0, 0, 0 };
    std::cout << "buff size:" << sizeof(buff) << std::endl;         //prints 3
    f(buff);
    f1(buff);
    return 0;
}
// g++ --std=c++11 test.cpp

NOTE: 输出如下:

buff size:3
void f(T) [with T = const char*]
f:buff size:8
void f1(T&) [with T = const char [3]]
f:buff size:3

A

Because arrays can not be passed by value as a function parameter. When you pass them by value they decay into a pointer.

In this function:

template <class T>
void f(T buff) {

T can not be char (&buff)[3] as this is a reference. The compiler would have tried char (buff)[3] to pass by value but that is not allowed. So to make it work arrays decay to pointers.

Your second function works because here the array is passed by reference:

template <class T>
void f1(T& buff) {

// Here T& => char (&buff)[3]

A

It is because arrays cannot be passed by value to a function. So in order to make it work, the array decays into a pointer which then gets passed to the function by value.

In other words, passing an array by value is akin to initializing an array with another array, but in C++ an array cannot be initialized with another array:

char buff[3] = {0,0,0};
char x[3] = buff; //error

So if an array appears on the right hand side of =, the left hand side has to be either pointer or reference type:

char *y = buff; //ok - pointer
char (&z)[3] = buff; //ok - reference

Demo : http://www.ideone.com/BlfSv

It is exactly for the same reason auto is inferred differently in each case below (note that auto comes with C++11):

auto a = buff; //a is a pointer - a is same as y (above)
std::cout << sizeof(a) << std::endl; //sizeof(a) == sizeof(char*)

auto & b = buff; //b is a reference to the array - b is same as z (above)
std::cout << sizeof(b) << std::endl; //sizeof(b) == sizeof(char[3])

Output:

4 //size of the pointer
3 //size of the array of 3 chars

Demo : http://www.ideone.com/aXcF5

C string

#include <iostream>

template<class T>
void f(T buff)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    std::cout << "f:buff size:" << sizeof(buff) << std::endl;       //prints 4
}

template<class T>
void f1(T &buff)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    std::cout << "f:buff size:" << sizeof(buff) << std::endl;       //prints 3
}

int main(int argc, char *argv[])
{
    f("abc");
    f1("abc");
    return 0;
}
// g++ --std=c++11 test.cpp

NOTE:

1、上述程序的输出如下:
void f(T) [with T = const char*]
f:buff size:8
void f1(T&) [with T = const char [4]]
f:buff size:4
2、在 f(T buff) 中，发生了array to pointer decay，它是pass-by-pointer，在 f1(T &buff) 中，是pass-by-reference，没有decay；

上述例子非常好的体现了: "Array-to-pointer decay 违背 static type safety"

Pass-by-reference example

code-examples Why is value taking setter member functions not recommended in Herb Sutter's CppCon 2014 talk(Back to Basics: Modern C++ Style)?

#include<array>
#include<string>

template<class C>
struct string_view
{
// known continuous memory containers:
    template<std::size_t N>
    string_view(const C (&arr)[N]) :
                    string_view(arr, arr + N)
    {
    }
};

这篇文章收录于basic_string_view章节了

C++ Type Erasure # Erasing down to a contiguous buffer of T#

template<class T>
struct array_view
{
    // the core of the class:
    T *b = nullptr;
    T *e = nullptr;
    // array constructor:
    template<std::size_t N>
    array_view(T (&arr)[N]) :
                    array_view(arr, N)
    {
    }

};