modernize-use-std-print

Converts calls to printf, fprintf, absl::PrintF and absl::FPrintf to equivalent calls to C++23’s std::print or std::println as appropriate, modifying the format string appropriately. The replaced and replacement functions can be customised by configuration options. Each argument that is the result of a call to std::string::c_str() and std::string::data() will have that now-unnecessary call removed in a similar manner to the readability-redundant-string-cstr check.

In other words, it turns lines like:

fprintf(stderr, "The %s is %3d\n", description.c_str(), value);

into:

std::println(stderr, "The {} is {:3}", description, value);

If the ReplacementPrintFunction or ReplacementPrintlnFunction options are left at or set to their default values then this check is only enabled with -std=c++23 or later.

Macros starting with PRI and __PRI from <inttypes.h> are expanded, escaping is handled and adjacent strings are concatenated to form a single StringLiteral before the format string is converted. Use of any other macros in the format string will cause a warning message to be emitted and no conversion will be performed. The converted format string will always be a single string literal.

The check doesn’t do a bad job, but it’s not perfect. In particular:

• It assumes that the format string is correct for the arguments. If you get any warnings when compiling with -Wformat then misbehaviour is possible.

• At the point that the check runs, the AST contains a single StringLiteral for the format string where escapes have been expanded. The check tries to reconstruct escape sequences, they may not be the same as they were written (e.g. "\x41\x0a" will become "A\n" and "ab" "cd" will become "abcd".)

• It supports field widths, precision, positional arguments, leading zeros, leading +, alignment and alternative forms.

• Use of any unsupported flags or specifiers will cause the entire statement to be left alone and a warning to be emitted. Particular unsupported features are:

  • The %' flag for thousands separators.

  • The glibc extension %m.

• printf and similar functions return the number of characters printed. std::print does not. This means that any invocations that use the return value will not be converted. Unfortunately this currently includes explicitly-casting to void. Deficiencies in this check mean that any invocations inside GCC compound statements cannot be converted even if the resulting value is not used.

If conversion would be incomplete or unsafe then the entire invocation will be left unchanged.

If the call is deemed suitable for conversion then:

• printf, fprintf, absl::PrintF, absl::FPrintF and any functions specified by the PrintfLikeFunctions option or FprintfLikeFunctions are replaced with the function specified by the ReplacementPrintlnFunction option if the format string ends with \n or ReplacementPrintFunction otherwise.

• the format string is rewritten to use the std::formatter language. If a \n is found at the end of the format string not preceded by r then it is removed and ReplacementPrintlnFunction is used rather than ReplacementPrintFunction.

• any arguments that corresponded to %p specifiers that std::formatter wouldn’t accept are wrapped in a static_cast to const void *.

• any arguments that corresponded to %s specifiers where the argument is of signed char or unsigned char type are wrapped in a reinterpret_cast<const char *>.

• any arguments where the format string and the parameter differ in signedness will be wrapped in an appropriate static_cast if StrictMode is enabled.

• any arguments that end in a call to std::string::c_str() or std::string::data() will have that call removed.

Options

StrictMode

When true, the check will add casts when converting from variadic functions like printf and printing signed or unsigned integer types (including fixed-width integer types from <cstdint>, ptrdiff_t, size_t and ssize_t) as the opposite signedness to ensure that the output matches that of printf. This does not apply when converting from non-variadic functions such as absl::PrintF and fmt::printf. For example, with StrictMode enabled:

int i = -42;
unsigned int u = 0xffffffff;
printf("%u %d\n", i, u);

would be converted to:

std::print("{} {}\n", static_cast<unsigned int>(i), static_cast<int>(u));

to ensure that the output will continue to be the unsigned representation of -42 and the signed representation of 0xffffffff (often 4294967254 and -1 respectively.) When false (which is the default), these casts will not be added which may cause a change in the output.

PrintfLikeFunctions

A semicolon-separated list of (fully qualified) function names to replace, with the requirement that the first parameter contains the printf-style format string and the arguments to be formatted follow immediately afterwards. Qualified member function names are supported, but the replacement function name must be unqualified. If neither this option nor FprintfLikeFunctions are set then the default value for this option is printf; absl::PrintF, otherwise it is empty.

FprintfLikeFunctions

A semicolon-separated list of (fully qualified) function names to replace, with the requirement that the first parameter is retained, the second parameter contains the printf-style format string and the arguments to be formatted follow immediately afterwards. Qualified member function names are supported, but the replacement function name must be unqualified. If neither this option nor PrintfLikeFunctions are set then the default value for this option is fprintf; absl::FPrintF, otherwise it is empty.

ReplacementPrintFunction

The function that will be used to replace printf, fprintf etc. during conversion rather than the default std::print when the originalformat string does not end with \n. It is expected that the function provides an interface that is compatible with std::print. A suitable candidate would be fmt::print.

ReplacementPrintlnFunction

The function that will be used to replace printf, fprintf etc. during conversion rather than the default std::println when the original format string ends with \n. It is expected that the function provides an interface that is compatible with std::println. A suitable candidate would be fmt::println.

PrintHeader

The header that must be included for the declaration of ReplacementPrintFunction so that a #include directive can be added if required. If ReplacementPrintFunction is std::print then this option will default to <print>, otherwise this option will default to nothing and no #include directive will be added.