I am new to C++17 and to std::string_view.
I learned that they are not null terminated and must be handled with care.
Is this the right way to printf() one?
#include<string_view>
#include<cstdio>
int main()
{
std::string_view sv{"Hallo!"};
printf("=%*s=\n", static_cast<int>(sv.length()), sv.data());
return 0;
}
(or use it with any other printf-style function?)
This is strange requirement, but it is possible:
std::string_view s{"Hallo this is longer then needed!"};
auto sub = s.substr(0, 5);
printf("=%.*s=\n", static_cast<int>(sub.length()), sub.data());
https://godbolt.org/z/nbeMWo1G1
As you can see you were close to solution.
int can lead to UB. Does printf support passing a size_t for the length of the string?
int is supported.* is what receives the precision. So, '.5s', sv.data() or '.*s', (int)sv.size(), sv.data() does the same thing (as long as size() doesn't overflow the int).
printf requires int value there so cast is needed. Star parameter doesn't have any modifiers to change expected type. It is possible to implement safe casting to int, but IMO it would be overkill.clip_cast.You can use:
assert(sv.length() <= INT_MAX);
std::printf(
"%.*s",
static_cast<int>(sv.length()),
sv.data());
If using google's Abseil library (https://abseil.io/) the absl::PrintF function can safely print a string view (either std::string_view or absl::string_view).
From the Abseil documentation of absl::PrintF at
https://github.com/abseil/abseil-cpp/blob/master/absl/strings/str_format.h
std::string_view s = "Ulaanbaatar";
absl::PrintF("The capital of Mongolia is %s", s);
The thing to remember about string_view is that it will never modify the underlying character array. So, if you pass a C-string to the string_view constructor, the sv.data() method will always return the same C-string.
So, this specific case will always work:
#include <string_view>
#include <cstdio>
int main() {
std::string_view sv {"Hallo!"};
printf("%s\n", sv.data());
}
string_view objects.
printfin c++ at all?string_viewcan be non-null terminated, but only if you create it as such."Hallo"has a null terminator, sosvwill as well."=%.*s=\n"... Just checked, that's correct.fmtlibrary it should easy to transform code which usesprintf.