blob: 5bbda9dd626eb9b2dc47489a5bf0173182b699eb [file] [log] [blame]
//===- FormatVariadic.h - Efficient type-safe string formatting --*- C++-*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements the formatv() function which can be used with other LLVM
// subsystems to provide printf-like formatting, but with improved safety and
// flexibility. The result of `formatv` is an object which can be streamed to
// a raw_ostream or converted to a std::string or llvm::SmallString.
// // Convert to std::string.
// std::string S = formatv("{0} {1}", 1234.412, "test").str();
// // Convert to llvm::SmallString
// SmallString<8> S = formatv("{0} {1}", 1234.412, "test").sstr<8>();
// // Stream to an existing raw_ostream.
// OS << formatv("{0} {1}", 1234.412, "test");
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/FormatCommon.h"
#include "llvm/Support/FormatProviders.h"
#include "llvm/Support/FormatVariadicDetails.h"
#include "llvm/Support/raw_ostream.h"
#include <cstddef>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
namespace llvm {
enum class ReplacementType { Empty, Format, Literal };
struct ReplacementItem {
ReplacementItem() = default;
explicit ReplacementItem(StringRef Literal)
: Type(ReplacementType::Literal), Spec(Literal) {}
ReplacementItem(StringRef Spec, size_t Index, size_t Align, AlignStyle Where,
char Pad, StringRef Options)
: Type(ReplacementType::Format), Spec(Spec), Index(Index), Align(Align),
Where(Where), Pad(Pad), Options(Options) {}
ReplacementType Type = ReplacementType::Empty;
StringRef Spec;
size_t Index = 0;
size_t Align = 0;
AlignStyle Where = AlignStyle::Right;
char Pad;
StringRef Options;
class formatv_object_base {
// The parameters are stored in a std::tuple, which does not provide runtime
// indexing capabilities. In order to enable runtime indexing, we use this
// structure to put the parameters into a std::vector. Since the parameters
// are not all the same type, we use some type-erasure by wrapping the
// parameters in a template class that derives from a non-template superclass.
// Essentially, we are converting a std::tuple<Derived<Ts...>> to a
// std::vector<Base*>.
struct create_adapters {
template <typename... Ts>
std::vector<detail::format_adapter *> operator()(Ts &... Items) {
return std::vector<detail::format_adapter *>{&Items...};
StringRef Fmt;
std::vector<detail::format_adapter *> Adapters;
std::vector<ReplacementItem> Replacements;
static bool consumeFieldLayout(StringRef &Spec, AlignStyle &Where,
size_t &Align, char &Pad);
static std::pair<ReplacementItem, StringRef>
splitLiteralAndReplacement(StringRef Fmt);
formatv_object_base(StringRef Fmt, std::size_t ParamCount)
: Fmt(Fmt), Replacements(parseFormatString(Fmt)) {
formatv_object_base(formatv_object_base const &rhs) = delete;
formatv_object_base(formatv_object_base &&rhs)
: Fmt(std::move(rhs.Fmt)),
Adapters(), // Adapters are initialized by formatv_object
Replacements(std::move(rhs.Replacements)) {
void format(raw_ostream &S) const {
for (auto &R : Replacements) {
if (R.Type == ReplacementType::Empty)
if (R.Type == ReplacementType::Literal) {
S << R.Spec;
if (R.Index >= Adapters.size()) {
S << R.Spec;
auto W = Adapters[R.Index];
FmtAlign Align(*W, R.Where, R.Align, R.Pad);
Align.format(S, R.Options);
static std::vector<ReplacementItem> parseFormatString(StringRef Fmt);
static Optional<ReplacementItem> parseReplacementItem(StringRef Spec);
std::string str() const {
std::string Result;
raw_string_ostream Stream(Result);
Stream << *this;
return Result;
template <unsigned N> SmallString<N> sstr() const {
SmallString<N> Result;
raw_svector_ostream Stream(Result);
Stream << *this;
return Result;
template <unsigned N> operator SmallString<N>() const { return sstr<N>(); }
operator std::string() const { return str(); }
template <typename Tuple> class formatv_object : public formatv_object_base {
// Storage for the parameter adapters. Since the base class erases the type
// of the parameters, we have to own the storage for the parameters here, and
// have the base class store type-erased pointers into this tuple.
Tuple Parameters;
formatv_object(StringRef Fmt, Tuple &&Params)
: formatv_object_base(Fmt, std::tuple_size<Tuple>::value),
Parameters(std::move(Params)) {
Adapters = apply_tuple(create_adapters(), Parameters);
formatv_object(formatv_object const &rhs) = delete;
formatv_object(formatv_object &&rhs)
: formatv_object_base(std::move(rhs)),
Parameters(std::move(rhs.Parameters)) {
Adapters = apply_tuple(create_adapters(), Parameters);
// Format text given a format string and replacement parameters.
// ===General Description===
// Formats textual output. `Fmt` is a string consisting of one or more
// replacement sequences with the following grammar:
// rep_field ::= "{" [index] ["," layout] [":" format] "}"
// index ::= <non-negative integer>
// layout ::= [[[char]loc]width]
// format ::= <any string not containing "{" or "}">
// char ::= <any character except "{" or "}">
// loc ::= "-" | "=" | "+"
// width ::= <positive integer>
// index - A non-negative integer specifying the index of the item in the
// parameter pack to print. Any other value is invalid.
// layout - A string controlling how the field is laid out within the available
// space.
// format - A type-dependent string used to provide additional options to
// the formatting operation. Refer to the documentation of the
// various individual format providers for per-type options.
// char - The padding character. Defaults to ' ' (space). Only valid if
// `loc` is also specified.
// loc - Where to print the formatted text within the field. Only valid if
// `width` is also specified.
// '-' : The field is left aligned within the available space.
// '=' : The field is centered within the available space.
// '+' : The field is right aligned within the available space (this
// is the default).
// width - The width of the field within which to print the formatted text.
// If this is less than the required length then the `char` and `loc`
// fields are ignored, and the field is printed with no leading or
// trailing padding. If this is greater than the required length,
// then the text is output according to the value of `loc`, and padded
// as appropriate on the left and/or right by `char`.
// ===Special Characters===
// The characters '{' and '}' are reserved and cannot appear anywhere within a
// replacement sequence. Outside of a replacement sequence, in order to print
// a literal '{' or '}' it must be doubled -- "{{" to print a literal '{' and
// "}}" to print a literal '}'.
// ===Parameter Indexing===
// `index` specifies the index of the parameter in the parameter pack to format
// into the output. Note that it is possible to refer to the same parameter
// index multiple times in a given format string. This makes it possible to
// output the same value multiple times without passing it multiple times to the
// function. For example:
// formatv("{0} {1} {0}", "a", "bb")
// would yield the string "abba". This can be convenient when it is expensive
// to compute the value of the parameter, and you would otherwise have had to
// save it to a temporary.
// ===Formatter Search===
// For a given parameter of type T, the following steps are executed in order
// until a match is found:
// 1. If the parameter is of class type, and inherits from format_adapter,
// Then format() is invoked on it to produce the formatted output. The
// implementation should write the formatted text into `Stream`.
// 2. If there is a suitable template specialization of format_provider<>
// for type T containing a method whose signature is:
// void format(const T &Obj, raw_ostream &Stream, StringRef Options)
// Then this method is invoked as described in Step 1.
// 3. If an appropriate operator<< for raw_ostream exists, it will be used.
// For this to work, (raw_ostream& << const T&) must return raw_ostream&.
// If a match cannot be found through either of the above methods, a compiler
// error is generated.
// ===Invalid Format String Handling===
// In the case of a format string which does not match the grammar described
// above, the output is undefined. With asserts enabled, LLVM will trigger an
// assertion. Otherwise, it will try to do something reasonable, but in general
// the details of what that is are undefined.
template <typename... Ts>
inline auto formatv(const char *Fmt, Ts &&... Vals) -> formatv_object<decltype(
std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...))> {
using ParamTuple = decltype(
return formatv_object<ParamTuple>(
} // end namespace llvm