C++26 标准新增了合约（Contracts）功能，为开发者提供了在函数声明中直接表达函数要求和保证的能力1。该特性通过 pre() 和 post() 语法替代传统的 assert() 方法，其中前置条件用于表达函数进入时应满足的条件，后置条件表达函数正常返回时应满足的条件1。这一设计使得调用者无需查看函数实现即可了解函数的具体要求，显著提高了 API 的文档化和可读性1。
合约系统支持四种评估语义：ignore（忽略）、observe（观察）、enforce（强制）和 quick-enforce（快速强制）1。编译时使用命令 g++ -std=c++26 -fcontracts -fcontract-evaluation-semantic=enforce 可启用该特性，目前仅 GCC 16 支持此功能1。后置条件中使用的值参数必须为 const 类型1。合约主要用于表达内部函数的正确性假设，不应用于输入验证和错误处理1。
The C++26 standard introduces a contracts system that allows developers to express function requirements and guarantees directly within function declarations using pre() and post() syntax 1. This feature replaces traditional assert() statements and enables clearer documentation of API expectations without requiring callers to examine the function's implementation 1.
Preconditions, specified with pre(), define conditions that must be satisfied when a function is entered, while postconditions, specified with post(), express conditions that must hold when the function returns normally 1. The contracts system supports four evaluation semantics: ignore, observe, enforce, and quick-enforce 1. When postconditions reference value parameters, those parameters must be declared as const 1.
Currently, only GCC 16 supports this feature, which requires compilation with specific options: g++ -std=c++26 -fcontracts -fcontract-evaluation-semantic=enforce 1. According to the specification, contracts should not be used for input validation and error handling; instead, they should express only the internal correctness assumptions of functions 1.
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