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void<\/span> f<\/span>(span<<\/span>int<\/span>><\/span> a) \/\/ BETTER: use span in the function declaration<\/span>\r\n{\r\n if<\/span> (a.length() <<\/span> 2<\/span>) return<\/span>;\r\n\r\n int<\/span> n =<\/span> a[0<\/span>]; \/\/ OK<\/span>\r\n\r\n span<<\/span>int<\/span>><\/span> q =<\/span> a.subspan(1<\/span>); \/\/ OK<\/span>\r\n\r\n if<\/span> (a.length() <<\/span> 6<\/span>) return<\/span>;\r\n\r\n a[4<\/span>] =<\/span> 1<\/span>; \/\/ OK<\/span>\r\n\r\n a[count -<\/span> 1<\/span>] =<\/span> 2<\/span>; \/\/ OK<\/span>\r\n\r\n use(a.data(), 3<\/span>); \/\/ OK<\/span>\r\n}\r\n<\/pre>\n<\/div>\n <\/p>\n
Fine! gsl::span<\/span> checks at run-time its boundaries. Additionally, the Guidelines Support Library has a free function at<\/span> for accessing the elements of an gsl::span.<\/span> <\/p>\n\n
void<\/span> f3<\/span>(array<<\/span>int<\/span>, 10<\/span>><\/span> a, int<\/span> pos) \r\n{\r\n at(a, pos \/<\/span> 2<\/span>) =<\/span> 1<\/span>; \/\/ OK<\/span>\r\n at(a, pos -<\/span> 1<\/span>) =<\/span> 2<\/span>; \/\/ OK<\/span>\r\n}\r\n<\/pre>\n<\/div>\n <\/p>\n
I know your issue. Most of you don’t use the Guidelines Support Library. No problem. It’s quite easy to rewrite the functions f<\/span> and f3 <\/span>using the container std:<\/span>:array<\/span> and the method std::array::at<\/span>. Here we are:<\/p>\n\n
\/\/ spanVersusArray.cpp<\/span>\r\n\r\n#include <algorithm><\/span>\r\n#include <array><\/span>\r\n\r\nvoid<\/span> use<\/span>(int<\/span>*<\/span>, int<\/span>){}\r\n\r\nvoid<\/span> f<\/span>(std::<\/span>array<<\/span>int<\/span>, 100<\/span>>&<\/span> a){\r\n\r\n if<\/span> (a.size() <<\/span> 2<\/span>) return<\/span>;\r\n\r\n int<\/span> n =<\/span> a.at(0<\/span>); \r\n\r\n std::<\/span>array<<\/span>int<\/span>, 99<\/span>><\/span> q;\r\n std::<\/span>copy(a.begin() +<\/span> 1<\/span>, a.end(), q.begin()); \/\/ (1)<\/span>\r\n\r\n if<\/span> (a.size() <<\/span> 6<\/span>) return<\/span>;\r\n\r\n a.at(4<\/span>) =<\/span> 1<\/span>; \r\n\r\n a.at(a.size() -<\/span> 1<\/span>) =<\/span> 2<\/span>;\r\n\r\n use(a.data(), 3<\/span>); \r\n}\r\n\r\nvoid<\/span> f3<\/span>(std::<\/span>array<<\/span>int<\/span>, 10<\/span>><\/span> a, int<\/span> pos){\r\n a.at(pos \/<\/span> 2<\/span>) =<\/span> 1<\/span>; \r\n a.at(pos -<\/span> 1<\/span>) =<\/span> 2<\/span>; \r\n}\r\n\r\nint<\/span> main<\/span>(){\r\n\r\n std::<\/span>array<<\/span>int<\/span>, 100<\/span>><\/span> arr{};\r\n\r\n f(arr);\r\n \r\n std::<\/span>array<<\/span>int<\/span>, 10<\/span>><\/span> arr2{};\r\n \r\n f3(arr2, 6<\/span>);\r\n\r\n}\r\n<\/pre>\n<\/div>\n <\/p>\n
The std:<\/span>:array::at<\/span> Operator will check at runtime its bounds. If pos >= size()<\/span>, you will get an std::out_of_range<\/span><\/a> exception. <\/span>Looking carefully at the spanVersusArray.cpp<\/span> program, you will notice two issues. First, the expression (1) is more verbose than the gsl:<\/span>:span version and second, the size of the std:<\/span>:array is part of the signature of the function f.<\/span> This is bad. I can only use f with the type std:<\/span>:array<int, 100><\/span>. In this case, the checks of the array size inside the function are superfluous. <\/span><\/p>\nTo your rescue, C++ has templates; therefore, it’s easy to overcome the type restrictions but stay type-safe.<\/p>\n
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\/\/ at.cpp<\/span>\r\n\r\n#include <algorithm><\/span>\r\n#include <array><\/span>\r\n#include <deque><\/span>\r\n#include <string><\/span>\r\n#include <vector><\/span>\r\n\r\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\r\nvoid<\/span> use(T*<\/span>, int<\/span>){}\r\n\r\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\r\nvoid<\/span> f(T&<\/span> a){\r\n\r\n if<\/span> (a.size() <<\/span> 2<\/span>) return<\/span>;\r\n\r\n int<\/span> n =<\/span> a.at(0<\/span>); \r\n\r\n std::<\/span>array<<\/span>typename<\/span> T::<\/span>value_type , 99<\/span>><\/span> q; \/\/ (4)<\/span>\r\n std::<\/span>copy(a.begin() +<\/span> 1<\/span>, a.end(), q.begin()); \r\n\r\n
![\"dental\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2018\/02\/dental.png\")
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