Using absl::MakeCheckOpString for Custom CHECK Macros in Abseil

Use absl::MakeCheckOpString from absl/log/internal/check_op.h to generate formatted failure messages that display operand values, enabling custom CHECK macros with the same type-safe formatting and low-overhead characteristics as the built-in Abseil logging checks.

When building custom assertion macros in C++, formatting rich failure messages that display actual operand values requires significant template machinery. The abseil-cpp library exposes absl::MakeCheckOpString, a utility function that powers the standard CHECK_EQ, CHECK_NE, and related macros, allowing developers to extend the CHECK family with custom logic while leveraging Abseil's optimized type detection and string formatting infrastructure.

How absl::MakeCheckOpString Works

The absl::MakeCheckOpString function is the core string-building routine that generates the expr (value1 vs. value2) message when a CHECK fails. Implemented in [absl/log/internal/check_op.h](https://github.com/abseil/abseil-cpp/blob/master/absl/log/internal/check_op.h) at lines 326-344, it is deliberately marked ABSL_ATTRIBUTE_NOINLINE to prevent code bloat in hot paths, with common instantiations declared extern (lines 46-66) to further reduce binary size.

The Three Core Components

The implementation relies on three cooperating parts:

• CheckOpMessageBuilder (lines 199-209): Holds a temporary std::ostringstream and concatenates the expression text, opening parenthesis, the two formatted values, and the closing parenthesis.
• MakeCheckOpValueString (lines 215-226): Streams a single value to the builder using the best available overload, prioritizing operator<<, then AbslStringify, or falling back to a placeholder.
• MakeCheckOpString (lines 326-344): Orchestrates the process by detecting whether operands are printable, invoking the builder, and returning a const char* suitable for the logging infrastructure.

Type Detection and Template Optimization

Heavy lifting for type resolution occurs in the detect_specialization namespace (lines 360-447). This machinery resolves ambiguous overloads, promotes integral types to 64-bit variants, and substitutes UnprintableWrapper for types lacking streamable representations. Instead of an O(n²) template explosion for every (T, U) pair, only distinct printable categories are instantiated, dramatically reducing compilation times and binary sizes.

Implementing a Custom CHECK Macro

To create a custom CHECK macro like MY_CHECK, you must wrap MakeCheckOpString in a template helper and follow the Abseil pattern of using a while loop with GetReferenceableValue to safely handle operands.

First, include the internal header and define a forwarder function:

#include "absl/log/internal/check_op.h"

namespace mylog {

template <typename T1, typename T2>
inline const char* MyCheckImpl(const T1& v1, const T2& v2,
                               const char* exprtext) {
  // Re-use the existing MakeCheckOpString machinery.
  return ::absl::log_internal::MakeCheckOpString(v1, v2, exprtext);
}

// Provide an overload for ints to avoid the unnamed-enum issue.
inline const char* MyCheckImpl(int v1, int v2, const char* exprtext) {
  return MyCheckImpl<int, int>(v1, v2, exprtext);
}

}  // namespace mylog

Next, define the macro following the ABSL_LOG_INTERNAL_CHECK_OP pattern:

#define MY_CHECK(op, v1, v2)                                           \
  while (const char* _absl_chk_res =                                   \
             mylog::MyCheckImpl(::absl::log_internal::GetReferenceableValue(v1), \
                               ::absl::log_internal::GetReferenceableValue(v2), \
                               "MY_CHECK: " #v1 " " #op " " #v2))    \
    ::absl::log_internal::ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS, true) \
  ::absl::log_internal::ABSL_LOG_INTERNAL_CHECK(_absl_chk_res).InternalStream()

The while loop ensures the check is evaluated once, and GetReferenceableValue prevents dangling references to temporary objects. The macro only builds the error string when the condition evaluates to true (the check fails).

Usage Example

int a = 5, b = 10;
MY_CHECK(<, a, b);   // Succeeds silently
MY_CHECK(==, a, b);  // Fails with: MY_CHECK: a == b (5 vs. 10)

This automatically supports any type that implements operator<<, AbslStringify, or Abseil's custom streaming protocols.

Key Files and Dependencies

Extending CHECK macros requires understanding these internal components:

• absl/log/internal/check_op.h: Contains MakeCheckOpString, CheckOpMessageBuilder, and the type detection logic.
• absl/log/internal/nullstream.h: Provides NullStream for code paths where logging is disabled.
• absl/log/internal/nullguard.h: Guards values before streaming to prevent use-after-free scenarios.
• absl/strings/has_ostream_operator.h: Metaprogramming utilities to detect operator<< support.
• absl/strings/has_absl_stringify.h: Detects AbslStringify support for user-defined types.

Summary

• absl::MakeCheckOpString in absl/log/internal/check_op.h generates failure messages showing operand values when CHECK conditions fail.
• The implementation uses CheckOpMessageBuilder and MakeCheckOpValueString to handle formatting, with detect_specialization logic to minimize template bloat.
• Custom macros must follow the while loop pattern with GetReferenceableValue and ABSL_LOG_INTERNAL_CONDITION_FATAL to maintain safety and performance characteristics.
• The system supports any printable type through operator<< or AbslStringify, with graceful degradation for unprintable types.

Frequently Asked Questions

What is the performance impact of using MakeCheckOpString?

The function is marked ABSL_ATTRIBUTE_NOINLINE to keep the fast path (successful checks) free of string formatting code. Common instantiations are declared extern (lines 46-66) to avoid duplicate template instantiations across translation units, minimizing binary size while preserving rich error messages only when assertions fail.

How does MakeCheckOpString handle types that cannot be streamed?

When a type lacks both operator<< and AbslStringify support, the detect_specialization machinery (lines 360-447) substitutes an UnprintableWrapper that renders a placeholder string. This prevents compilation failures while still logging the expression text and the fact that values could not be displayed.

Why is GetReferenceableValue used in CHECK macros?

GetReferenceableValue ensures that temporary objects and expressions are properly bound before being passed to the comparison and formatting functions. This prevents dangling references and use-after-free bugs that could occur if operands were evaluated multiple times or stored as references to temporaries.

Can I use MakeCheckOpString with custom comparison operators?

Yes. MakeCheckOpString is agnostic to the comparison logic itself—it only formats the operands and expression text when the check fails. You can implement any custom predicate (e.g., checking floating-point tolerance) and use MakeCheckOpString to generate the diagnostic message when your predicate returns false.

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