Qt - LNK2019 在类的成员函数中使用命名空间的函数时

我有一个包含函数声明的命名空间Project\test\compare_structs.h：

#ifndef COMPARE_STRUCTS_H
#define COMPARE_STRUCTS_H

#include <QtTest/QtTest>
#include "include/Utils/containers.h"

namespace compare_structs {
    void compareRawData(RAW_DATA actual, RAW_DATA expected);
    ...
}

#endif // COMPARE_STRUCTS_H

我在里面定义了函数Project\test\compare_structs.cpp：

#include "compare_structs.h"

void compare_structs::compareRawData(RAW_DATA actual, RAW_DATA expected)
{
    ...
}

我还将.h和.cpp文件添加到 Qts.pro文件中：

test {
HEADERS +=  ...
            test/sql_db_test.h \
            test/compare_structs.h \

SOURCES +=  ...
            test/sql_db_test.cpp \
            test/compare_structs.cpp \
}

现在，我#include的compare_structs.h一类.h，并调用它的内部.cpp文件：

Project\test\sql_db_test.h：

#ifndef SQL_DB_TEST_H
#define SQL_DB_TEST_H

...
#include "test/compare_structs.h"

class Sql_DB_Test : public QObject
{
    ...

private slots:
    ...
    void add_get_raw_data_test();
    ...
};

#endif // SQL_DB_TEST_H

Project\main\test\sql_db_test.cpp：

#include "sql_db_test.h"

void Sql_DB_Test::add_get_raw_data_test()
{
    ...
    compare_structs::compareRawData(rawBack, raw); //LNK2019
}

错误信息：

sql_db_test.obj:-1: Fehler: LNK2019: Verweis auf nicht aufgelöstes externes Symbol ""void __cdecl compare_structs::compareRawData(struct RAW_DATA,struct RAW_DATA)" (?compareRawData@compare_structs@@YAXURAW_DATA@@0@Z)" in Funktion ""private: void __thiscall Sql_DB_Test::add_get_raw_data_test(void)" (?add_get_raw_data_test@Sql_DB_Test@@AAEXXZ)".

免费翻译：unresolved externel symbol ""void __cdecl compare_structs::compareRawData(struct RAW_DATA,struct RAW_DATA)" (?compareRawData@compare_structs@@YAXURAW_DATA@@0@Z)" in function ""private: void __thiscall Sql_DB_Test::add_get_raw_data_test(void)" (?add_get_raw_data_test@Sql_DB_Test@@AAEXXZ)".

导致链接器错误的原因是什么？