error: macro "assert" passed 2 arguments, but takes just 1

Question

vector<string> foo(vector<string> s) { return s; }
assert(foo(vector<string>{"hello", "world"}) ==
         vector<string>{"hello", "world"});

error: macro "assert" passed 2 arguments, but takes just 1
error: ‘assert’ was not declared in this scope

maybe define assert in gcc 11.1.0

#  define assert(expr)                          \
     (static_cast <bool> (expr)                     \
      ? void (0)                            \
      : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))

compiler flag is

 -Wall -std=c++20

Silvio Mayolo · Accepted Answer · 2021-10-31 01:55:20Z

6

The preprocessor only has a primitive understanding of C++'s syntax, and in particular it sees any commas not enclosed in parentheses as argument separators. There are two commas in your assert call, and only one is enclosed in parentheses, so the macro thinks it's getting two arguments as follows

foo(vector<string>{"hello", "world"}) == vector<string>{"hello"
"world"});

Wrap the expression in parentheses to prevent this.

// Note: Double parens
assert((foo(vector<string>{"hello", "world"}) ==
        vector<string>{"hello", "world"}));

answered Oct 31, 2021 at 1:55

Silvio Mayolo

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